Iron Valiant is ridiculously hard to check, and I'll list a few faux checks before the real ones. I'll clarify as well that I won't talk about Tera that much because, yes, Tera Poison Poison Jab Garchomp can handle Iron Valiant but that's cheap. Though I'll point out relevant Tera types that have been born out of a need to check Iron Valiant.
1. Faux Checks
Some Pokemon can purport to be Iron Valiant checks, but don't really cut it in the grand scheme of things. Okidogi seems amazing, until you remember who Iron Valiant's grandparents are. One Psyshock and it's done. For this reason, Tera Psychic is Okidogi's most common Tera Type, because Dark just isn't able to take on the onslaught of Iron Valiant for obvious reasons. Assault Vest Heatran doesn't seem too bad but it's too vulnerable to mixed sets, or the niche option of Focus Blast. Assault Vest Iron Hands is technically able to EV to live Timid Tera Fairy Moonblast, but you're giving up enough EVs (124 + 252 to be precise) that you're doing a lot less damage and/or getting outsped spored by Brute Bonnet. Iron Bundle seems decent enough, but most Iron Valiant are either bulky enough to survive Hydro Pump, or you're gambling on not only the miss but on a measly 43% chance to OHKO. Choice Specs Iron Bundle can KO it with Ice Beam, but for several reasons I won't get into here, the Choice Specs set is poorly positioned into this meta. It's also very weak into Iron Valiant paired with speed control, as it can be knocked out by spread damage like Expanding Force or Tera Fairy Dazzling Gleam. That said, the forum kickoff winning team had an Iron Bundle with 116 HP / 252 SpD. This was confirmed to be tech for Iron Valiant, as it can live a timid Moonblast. It's a roll but whatever, everything with Iron Bundle is a roll. And, to be honest, if this is the level of counter teaming that's needed for Iron Valiant, it might be more suspect worthy than I thought.
2. Thundurus
Ironically, a bestie can be a hater too. Thundurus slows Iron Valiant down significantly with Thunder Wave, and turns it into a Kirlia with Eerie Impulse. Wildbolt Storm won't OHKO it, (even with a Modest nature and a Life Orb, it's a 25% chance on bulkless Iron Valiant) but the paralysis chance and damage certainly make Iron Valiant sad if anything. Thundurus can also use Electric Terrain, which makes Iron Valiant faster but also removes Psychic Terrain, preventing it from using Expanding Force and opening it up to opposing Fake Out and Thunder Wave.
3. Volcarona
Volcarona is a rare Pokemon that resists both Fairy and Fighting and isn't weak to Psychic either. After a Quiver Dance it's taking very little damage from any move Iron Valiant tends to use, besides Psyshock. In screens it's able to reliably set up in front of Iron Valiant, and at +2 it's able to Giga Drain Iron Valiant for an OHKO that also grants it massive healing. That said, it hasn't been super common in the shift 2 metagame, but I'm sure it will always be good as it's such a strong setup sweeper with a good defensive profile, as long as Tyranitar can be dealt with.
4. Excadrill
I considered putting this in the faux checks section but I think it's definitely better into the Iron Valiant matchup than something like Dogi so long as Sand is up, so I need to give it some respect. Excadrill in sand is able to KO Iron Valiant with Iron Head, as long as they aren't a bulky Booster Speed variant. Vacuum Wave from Iron Valiant does do annoyingly high damage to Excadrill, but they're usually Tera Ghost and they don't die to it unless you're using Tera Stellar Choice Specs or something. They also tend to use Focus Sash though, so that KOing wouldn't even matter. However, Excadrill's main issue is that it is very reliant on Sand being up, and if it goes down for any reason it's suddenly much slower. Opposing Tailwind also has the same effect, which allows Iron Valiant to threaten a KO on Tyranitar, or to bring Excadrill down to 1 HP/KO if they're a Life Orb variant. That said, it's still a pretty good option into Iron Valiant that aren't paired with priority Tailwind setters.
5. Armarouge
Armarouge resists all of Fairy, Fighting, and Psychic. It's not
great on its own but indeed, most Iron Valiant are not doing too much to this thing. That said, it can't exactly switch in on Iron Valiant. Once, a player tried that against me and was rudely awakened to the fact it takes over half from Tera Fairy Moonblast, and even at max HP it's about a 90% chance to 2HKO. Though at the end of the day, it's still a decent check until the day Iron Valiant starts running Knock Off. As an Expanding Force user, well, let's just say there are more sample teams with Indeedee paired with Iron Valiant than Armarouge.
6. Metagross
Metagross is sooooooooooooooooooo good. It's definitely one of the most underrated things in this format. With an Assault Vest it's not only a wonderful Iron Valiant check but it can handle Iron Hands and Okidogi nicely as well. Its physical bulk lets it take a Close Combat decently well and most Iron Valiant don't run Knock Off. Even if they did, it'd take a Choice Band and Tera Stellar for it to KO. In exchange, Metagross murders it with Heavy Slam, and Sash sets are pinned by Bullet Punch. Overall if your team struggles against Iron Valiant I think Metagross is one of the easiest adds, its main drawback is that it competes with things like Iron Hands for Assault Vest, sadly.
7. Iron Valiant
I swear whenever I fight Iron Valiant it feels so much stronger than when I use it. It's good and takes a lot of KOs for me, but then suddenly when I'm fighting it it's doing infinite damage and I can't stop it and it's so scary. What do you mean my Primarina in Light Screen takes half from Moonblast. Stop it don't kill my Garchomp. Stop winning speed ties. Don't Encore me. Don't Encore me. Don't Encore me. Don't Encore me
Roaring Moon
Kyurem-W
Zekrom
