Stalling out a Steel type is a very difficult task indeed. Steel is the top defensive type in the game, with 11 resistances and immunities to Poison and Sandstorm damage. It also packs defensive behemoths like Skarmory and Ferrothorn. However, don't worry if Offensive is your preferred playstyle - you're still well catered for with heavy-hitters in Metagross, Lucario, Genesect, and more, many of whom bring alternative STABs to offset Steel's poor offensive coverage. MonoSteel has its drawbacks, but a well built MonoSteel team - whether offensive or defensive - is a force to be reckoned with.
 Notable Members
Skarmory is one of the few Steel-types neutral to the Fighting-type. Coupled with its high defense stat, it is usually the first resort against most physical Fighting-type Pokémon. Honestly, though, if there's a Physical attacker on your opponent's team, chances are Skarmory can block it, setup Spikes all over it, and Whirlwind it out if it tries to setup, with Roost for good measure. As with many walls, Taunt and Burn hurt it, but sometimes Skarmory is the one doing the Taunting.
Another Steel type neutral to Fighting, Forretress still maintains its crown as best hazard setter. It can set up all 3 types of entry hazards, and Rapid Spin them away (although that's not as vital for a Mono Steel team). Its typing grants it a slew of resistances with only 1 weakness (albeit a big one). Its menial attack power often leaves it prone to becoming set up fodder, so you may choose to equip a Red Card to act as a one time stop. It also hates Taunt with all its soul.
Ferrothorn is one of the best defensive Pokémon around. It can stop most offensive Water types cold, and many special attackers are resorting to Hidden Power Fire in an effort to deal with this thing. It's not useless if Taunted either - although it has only marginally better Attack than Forretress, it has 2 STAB very high powered STAB moves in Power Whip and Gyro Ball - due to its appalling speed, Gyro Ball will almost always get the full 150 power. It can also set up Stealth Rock and Spikes, and lend support with Leech Seed or Thunder Wave.
Back in Gen 4, Heatran was OU's most popular Pokémon, and for good reason. Its excellent stat distribution allows it to fill a number of roles on your team. It can be a supporter with Stealth Rock and Roar, break stall with Toxic and Taunt, be a staller with the infamous Tormentran set, be a revenge killer with a Choice Scarf set, or blow holes into the opponent's team with a Choice Specs set. It can also use the Fire type attacks aimed at its fellow Steel types to power up its Fire STAB, courtesy of Flash Fire, which provides valuable defensive synergy. Heatran is MonoSteel's best weapon against MonoFire, since it can soak up their Fire moves and KO most Fire Pokémon with Earth Power. For MonoFire or MonoSteel teams, Heatran is often the glue that holds them together.
Team Plasma set out to revive and improve Genesect. Mission accomplished. Its incredible movepool includes gems like the BoltBeam combination, Flamethrower, Giga Drain, Iron Head and Bug Buzz. Those 120/120 attacking stats make it very difficult difficult to wall, and even if you do wall it, it can just U-turn out to a teammate. Although it's commonly seen running a Choice Scarf lead set, it can happily run an all out attacker Expert Belt set (which people more often than not mistake with the ever so common scarfer), a Rock Polish sweeper set, or even a Choice Band set if need be. Genesect is a very dangerous and threatening Pokémon that all teams need to prepare for.
Although it's not the top tier threat it was in Gen 3 and 4, Metagross is still worth considering. It has the highest Attack stat of all Steel types, and the moves to use it. Meteor Mash does high damage to anything that doesn't resist it; the Pokémon that do are badly hurt by Earthquake, Hammer Arm, ThunderPunch, Ice Punch and Zen Headbutt. Metagross is one of MonoSteel's best weapons to use against MonoFighting teams; with its colossal base 130 Defense and STAB Zen Headbutt, it can swiftly end most physical Fighting types. It does struggle against bulky Water and Ground types though.
One of the best Pokémon with base 100 all-round stats, Jirachi is one of MonoSteel's greatest assets. Its vast movepool and sizeable stats mean it can run a multitude of sets, making it extremely unpredictable. For just a few examples of what it can do, it can support with Wish, infuriate opponents with Thunder Wave/Body Slam and Iron Head, be a revenge killer with a Choice Scarf set, or sweep with either a Calm Mind + 3 attacks set or a SubCM set. And while Metagross struggles somewhat with bulky Water and Ground types, Jirachi doesn't – it gets Thunderbolt and Water Pulse to put them in their place. It also gets STAB Psychic and Zen Headbutt to use against MonoFighting teams, and STAB Psyshock to break special walls like Blissey and Gastrodon. Its only real drawbacks are that before boosting it will struggle to muscle through some dedicated walls, and it has a chronic case of 4 moveslot syndrome.
If you need a bulky supporter, Bronzong is a Pokémon you should definitely consider. With Stealth Rock, dual screens, and Trick Room, it's an ideal Pokémon for giving your sweepers a helping hand. Since permanent sun and rain don't exist in Monotype, Bronzong can utilise Rain Dance to help Empoleon, while also softening hits from strong Fire-type moves that most Steel-types hate. Its excellent bulk is improved with Levitate, giving it only one weakness, which is negated in rain (although it does trade it off for a pseudo Water weakness). Offensively it is quite limited, although Gyro Ball will do reasonable damage thanks to Bronzong's terrible speed.
Scizor is one of OU's elite Pokémon. It just has everything it needs – a STAB priority move in Bullet Punch boosted by Technician, the strongest U-turns in the game, good bulk, excellent attack power, and the ability to check some notable threats. Its movepool is rather limited – beyond its STABs and perhaps Swords Dance it doesn't have much to use. Most sets will be either a standard Choice Band set or involve setup with Swords Dance and sometimes Roost. Those sets are all it needs, however, to be a solid MonoSteel choice.
Empoleon is one of MonoSteel's best weapons against MonoGround and MonoFire teams. With its good bulk backed with strong STAB water moves, it can be played offensively or defensively. It can lay Stealth Rock and shuffle teams around with Roar and Yawn, blow holes in the opponent's team with Choice Specs, or sweep with Agility. It may have fallen from OU, but Empoleon's not to be underestimated.
Breaking the tradition for most Steel types, Lucario focuses entirely on offence. With its 110/115 attack stats and Swords Dance and Nasty Plot, Lucario can sweep from either side. Its mediocre speed doesn't let it down too much since it has ExtremeSpeed on the physical side and Vacuum Wave on the special side. It also has STAB Bullet Punch, to hurt faster Ghost types - it also OHKOs Terrakion at +2. Besides those priority moves, its movepool is vast, allowing it to hit everything for neutral damage. This means very little is safe from Lucario. However, while it can bring the pain it certainly can't take it - most strong neutral moves will likely knock it out. Even resisted hits will quickly wear it down.
With a slew of resistances, excellent special attack, and good bulk, Magnezone is rather like a special Metagross. It gained Volt Switch in BW, which allows Magnezone to scout switch ins and grab momentum. Its ability is its USP - it can trap and kill many Steel types like Ferrothorn and Skarmory with Magnet Pull, which otherwise might stop one of your physical sweepers. The absence of Team Preview in Monotype helps Magnezone too. In terms of sets, it can run a Substitute + 3 Attacks, Choice Scarf, Choice Specs, and even dual screens. It does struggle with bulky Ground types though.
Like Lucario, Bisharp breaks the trend for the Steel types by going on the offensive. It is pretty one dimensional, since it can only really run a Swords Dance set, and despite its good Attack stat, many of Bisharp's moves have low base power. It compensates for that with near perfect neutral coverage with STAB Sucker Punch and Brick Break. It can run an excellent Sub SD set with Substitute, Swords Dance, Sucker Punch and Brick Break, which can break stall teams almost by itself. It probably won't be your first choice, but it's worth considering.
This match up is heavily slanted towards MonoSteel. They resist MonoNormal's STABs, and Swords Dance Lucario can handle MonoNormal teams almost by itself, using Ice Punch or Stone Edge for the Normal/Flying types. The only Normal types that really give Steel types trouble are Sawsbuck with Jump Kick and Nature Power (which becomes Earthquake), Togekiss with Fire Blast and Aura Sphere, and Staraptor which has Close Combat. Skarmory walls any Normal-type Physical attacker, while Calm Mind Jirachi deals with Togekiss and Staraptor without much trouble.
MonoFire is a very tough matchup for MonoSteel. Lots of Steel types are X4 weak to Fire, and Infernape can hit all Steel types for super effective damage, so it can easily sweep MonoSteels. MonoSteel does have Empoleon though, so it's not all one-sided, and Bronzong can weaken their moves with Rain Dance, although it will have a lot of trouble switching in to do it. Heatran, one of MonoSteel's greatest assets, also regularly features on MonoFire teams as one of MonoSteel's most powerful enemies. Most of the time, in MonoFire vs MonoSteel, MonoFire will win.
This match up is neutral, since both MonoSteel and MonoWater have Pokémon that can damage the other's type. MonoSteel has Magnezone and Ferrothorn; MonoWater gets Keldeo and Gastrodon. Although Water-types resist Steel type moves, all Steel types that see OU are hybrids (bar Registeel), and often have alternative attacks to use; meanwhile, Steel-types that don't resist Water are often capable of tanking their hits anyways. MonoSteel vs MonoWater will usually be a match determined by the player's skill, since neither type has a significant advantage.
A strong matchup in favour of Steel types, since almost every Steel type resists Grass moves, with many packing a X4 resistance to them. The only Pokémon MonoGrass teams pack which cause trouble for MonoSteel teams are Breloom, Virizion and Sawsbuck, who have Swords Dance and Fighting STAB or, in the reindeer's case, non-STAB Jump Kick and Earthquake to abuse. If MonoSteel teams pack Metagross or Skarmory though, even those aren't very annoying. If Metagross runs 252 HP EVs, it can survive a +2 Mach Punch from Technician Breloom and a +2 Close Combat from Virizion and revenge kill them both with a super-effective STAB Zen Headbutt. Skarmory can do the same with STAB Brave Bird.
On paper, this match up should be pretty one sided, since Electric-types resist Steel moves. However, it's not that clear cut. Most Electric types are quite fragile, and strong neutral moves quickly wear them down. There are 4 Pokémon on MonoElectric teams that give MonoSteel major trouble: Thundurus-T with Focus Blast and its brutal Special Attack; its alternate form Thundurus with the same benefits; Zapdos with its great bulk, Roost and Heat Wave; and Rotom-H, who resists most of the moves Steel types use and packs STAB Overheat to turn most Steel types into molten slag. However, all 3 are weak to Stealth Rock, and there is no Electric type Rapid Spinner. Hidden Power Ice can hurt the first 2, while Stone Edge can cripple all of them. Once Agility Empoleon has set up, it can OHKO them all with Ice Beam and Hydro Pump respectively, and can outrun them if they aren't using a Choice Scarf.
On paper, this match up looks very one sided, and that is absolutely true. Ice-type Pokémon are weak to Steel moves, most of them are weak to Stealth Rock, and their only Rapid Spinners are Cryogonal, Cloyster, and the laughable Delibird. The only Pokémon MonoIce teams pack that seriously threaten MonoSteel are Mamoswine, Kyurem-B, and Walrein. Scizor can handle Mamoswine as well as Kyurem-B after Stealth Rocks pretty easily with its Technician boosted Bullet Punch. Walrein is a major problem, stalling MonoSteels mercilessly with the combination of Substitute and Protect--however, Skarmory can Whirlwind it out if it sets up a sub (unless it gets Roared out first). Assuming no major hax occurs and equally skilled players, MonoSteel will usually win against MonoIce.
Facing a type you're weak to is always going to be difficult. As mentioned earlier, Infernape can sweep some MonoSteel teams almost by itself. Breloom can incapacitate an opponent with Spore, set up Swords Dance and wreak havoc with its Technician boosted Mach Punch. Don't despair, though; MonoSteel has a few tricks up its sleeves. Metagross, Jirachi, Bronzong and Skarmory all have STABs to hit most Fighting types for super effective damage, and are very bulky to boot. MonoSteel may not be going in with the best odds, but don't count it out.
At first glance, you might think MonoSteel would breeze through MonoPoison teams. However, it's not that simple. Nidoking and Nidoqueen are major problems, with STAB Earth Power and Flamethrower, both boosted to terrifying levels by Sheer Force. They also pack Tentacruel, who resists Steel type moves and can cripple physical attackers with Scald, and Toxicroak who has Fighting STAB. SubCM Jirachi easily beats Tentacruel, and Skarmory can handle physical Toxicroak without any worries. Metagross can also stamp both with an Earthquake. Once the aformentioned problematic Pokémon are out of the way it's plain sailing.
MonoGround has a good advantage over MonoSteel. On the physical side they have Landorus-T, Dugtrio and Mamoswine; on the special side they have Landorus, Nidoking and Nidoqueen. They also pack one of the best special walls in OU: Gastrodon. Gastrodon can cripple physical attackers with Scald, and smash up foes with a STAB Earth Power, hitting many Steel types on their weaker Special Defence stat. MonoSteel can still put up a fight though, since they can sweep with Genesect and Empoleon with Ice Beam and Surf/Hydro Pump, as well as wall with Ferrothorn, Skarmory, and Bronzong. Even with those Pokémon though, it's an uphill struggle against MonoGround.
Despite Steel resisting Flying moves, this match up is pretty neutral. MonoFlying includes Gliscor, Landorus, and Landorus-T, all troublesome Pokémon for MonoSteel. Genesect is your greatest asset, as with a speed boost either from Scarf or Rock Polish, it can outspeed and sweep with Ice Beam or Thunderbolt. Most Flying types are weak to Stealth Rock and have no spinners, so laying Stealth Rock will give a MonoSteel team the edge--beware of Magic Bounce Xatu, however, as it might just set up those rocks on your side of the field!
Again, this match up is pretty neutral since both sides have Pokémon that badly threaten the other. Alakazam and Reuniclus both have Focus Blast to smite Steel types, and MonoPsychics can also resort to Victini, a Pokémon that strikes fear into the hearts of all Steel types bar Heatran with its STAB V-create. Slowbro can also use Fire Blast to injure the Steel types, and as a courtesy of its Water typing, Heatran will hardly ever switch in to try picking up a Flash Fire boost. However, Genesect and Scizor have strong Bug STABs to beat down the Psychic types. Quite a few Psychic types are frail as well, and pure Psychic types lack resistances, meaning they can be worn down by strong neutral hits.
You'd think Steel would have all the advantages here, but you'd be forgetting a few key Pokémon. Heracross with its STAB Close Combat and 2 great abilities in Guts and Moxie make it very threatening indeed. Then there's Volcarona, one of the most dangerous sweepers in OU. If that gets a Quiver Dance set up...well, you'd better have a Heatran. If it's packing Hidden Power Ground though, even that won't save you. Bar those aforementioned Pokémon, most of the time it won't be too difficult for MonoSteel if you can keep Stealth Rock on the field. That's easier said than done though, since MonoBug includes Forretress and Armaldo, and MonoSteel has no Spinblocker. So despite being at a disadvantage, due to those 2 Pokémon, the matchup is much closer than you'd think.
The other type weak to Steel. Steel has a pretty easy time here. Even though MonoRock teams can abuse permanent Sandstorm to boost their special defence, it doesn't help at all against physical attackers. But what about Terrakion? True, Terrakion can cause a lot of damage with its brutal attack and STAB super effective Close Combat. But Scizor and Metagross can handle it with Bullet Punch before it gets out of control--and, indeed, almost all Rock-types are 2HKOed by a Banded Scizor's Bullet Punch. Rhyperior could be more problematic due to its great bulk, Solid Rock, and its Ground STAB. Fortunately, Empoleon can handle it (and oftentimes the rest of the MonoRock) with its Water STABs. MonoRock teams will have to pray for a lot of hax to win.
Pure Ghost types are cursed with low powered STABs, and the more offensive Ghost types are cursed with low bulk: Gengar and Mismagius are KOd by a light breeze, and the more bulky ones like Dusclops have no reliable recovery. However, MonoGhost includes Golurk and Chandelure, who have very threatening alternative STABs to hurt Steel types. MonoSteel has Bisharp though, so it's not one sided. Neither side has a significant advantage.
You'd think Steel would have all the advantages here, which is sadly untrue. Many Dragon types learn Fire and/or Ground moves, with some (Dragonite, Haxorus and Hydreigon) learning Fighting moves as well. Dragon types also tend to be much quicker than Steel types, and they're not going to click Outrage once they know you have a MonoSteel team. Try to switch your bulky Steel type into Hydreigon and Salamence, and you'll get a rousing round of Chestnuts Roasting on an Open Fire. Choice Scarf Genesect is MonoSteel's best weapon against them - it outruns most Dragon types and packs Ice Beam to smite them. However, it will have trouble switching in to do it, since strong neutral hits and hazard damage quickly wear it down. MonoDragon also has Kingdra, who can survive most of Genesect's assaults and obliterate it with STAB Hydro Pump. Garchomp, another one of MonoDragon's assets, can sweep MonoSteel teams whole with its STAB Earthquakes. It only really worries about Bronzong and Skarmory. If it packs Fire Blast or Fire Fang, it doesn't even care about them. It's a very close match up to call - Scarf Genesect can make it tricky for the Dragons, but if Genesect's down MonoSteel teams will struggle.
This is a pretty even match up. MonoDark has Scrafty, Houndoom and Krookodile; MonoSteel has Lucario, Scizor and Genesect. MonoDark type teams will have trouble muscling through more defensive MonoSteel teams; offensive MonoSteel teams may have trouble finding time to get a boost in, though Scizor can often wreck the frailer Dark-types with Bullet Punch. It comes down to 2 things: which team is better built, and who is the better player. It's as simple as that.
This matchup rather depends on what style the players are using. If it's offensive vs defensive, once the offensive team breaks through the main core (easier said than done!) they've usually won. If it's offensive vs offensive, whoever loses their main sweeper first is generally going to lose the match. If it's defensive vs defensive, it's pretty much a stale mate. Heatran is a major component of any MonoSteel battle, as it sees use on every type of MonoSteel team. Steel vs Steel is a neutral match up, overall.