Disclaimer: this is a lot and I might change my opinion later because I'm tired right now
Right away, I feel it is necessary to (attempt to) disern my opponents sets. I think that Iron Bundle is Booster Energy that will boost its Speed stat, since that looks fine on this type of team and allows Flutter Mane to hit hard with Life Orb or Choice Specs. However, it is not impossible that Theorymon has instead opted for a Choice Specs Iron Bundle set with Speed-boosting Booster Energy Flutter Mane. I'm going to assume Heatran is a Stealth Rock set, since I honestly think Rocks are super useful in BSS and its the only thing that has access to the move on his team. I wouldn't be surprised if this Thundurus-Therian set is Choice Specs or Assault Vest, as there are better speed control options on his team. Regarding which of these it is, I honestly don't know, as both have merit on this type of build. I'm inclined to assume Assault Vest because I feel his team lacks a strong Electric-Type answer (which Assault Vest Thundurus-Therian can blanket check). Urshifu is probably Puching Glove or Sash, both seem fine on this team. I'm inclined to assume Focus Sash because this team has mostly Special Attackers, meaning that Pokémon carrying Rocky Helmet or with the Rough Skin ability become a lot less useful into it, and as a result would not be Theorymon's opponent's first choice to bring into the Battle. In summary, I assume the team looks something like
this.
My Lead:
Lead Reasoning: While Stealth Rock isn't too useful into this team, Ting-Lu can Whirlwind an Iron Bundle lead (albeit it will need to tera to do so, but its worth not letting Iron Bundle get a free Substitute) and can also prevent Dragon Dance Dragonite from sweeping with Whrilwind until late-game. Furthermore, Ting-Lu would force Heatran to burn its Tera, and even then Heatran doesn't particularly want to eat several Ruinations.
My Second Pokémon:
Second Pokémon Reasoning: Speed control + something that beats Dragon Dance Dragonite? Yes please! Multiscale shouldn't be that difficult to break, as worst case scenario you sacrifice Urshifu-Rapid-Strike to break it. If they stay in and Dragon Dance, then great! They'll be forced to either Extreme Speed or Tera Blast Flying you next turn (as they would fear Ice Punch), not allowing them the oppertunity to use Roost. In addition, Flutter Mane should also be able to take care of Iron Bundle, which poses a huge threat to the team as we lack a Pokémon that would otherwise be able to beat it.
My Third Pokémon:
Third Pokémon Reasoning: As I mentioned above, Urshifu-Rapid-Strike's priority can save the team in a pinch. In addition, it resists Magma Storm and can pick off a weakened Heatran without forcing Flutter Mane to, as Flutter Mane doesn't want to be Taunted. Furthermore, it outspeeds a decent number of Pokémon on Theorymon's team, which means it can deal big damage with Close Combat.
Pokémon I Excluded:

,

,
Exclusion Reasoning:

While it could Spore something, Amoonguss is at risk of Iron Bundle setting up a Substitute freely on its Spore attempt, which is a 50/50 I'd like to avoid because it is, in my opinion, disadvantageous to the Amoonguss user. Furthermore, with Thundurus-Therian, Dragonite, and Flutter Mane on Theorymon's team, Amoonguss can become a huge liability. I think there is a little merit to bringing it, as it will hard stop any Urshifu-Rapid-Strike variant not carrying Ice Punch (which I think is the case here, as personally I'd run Swords Dance on this team assuming my prior assumptions about Theorymon's team are correct), but that merit is overshadowed by the cost of using one of my 3 slots for it.

It doesn't really beat anything on Theorymon's team, except for Dragonite assuming it's a Bulky Dragon Dance set.

Other Pokémon are just more valuble into this matchup.