How high can one Pokemon boost its stats in one turn with no assistance?

Question

I'm making a team with costar flamigo, and I want it to have a partner that can boost its own stats really high in one turn so flamigo can copy it later.
I'm looking for a combo of moves, abilities, items, etc that can boost stats as much as possible in one turn. Thanks in advance!

Comments

Belly drum maxes the user's physical attack. Justified and steam engine work multiple times if the Pokemon gets hit by a multi-hit move.

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thanks!

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Since you need to be in doubles anyway for Costar to work, self-targeting a water move into Steam Engine + Weakness Policy is definitely a contender. Simple Beam + Shell Smash also sounds interesting, though I'm pretty sure Simple Beam was dexited (correct me if I'm wrong), so that's only an option if you're playing NatDex doubles.

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Simple Beam exists in the game, but only on Golduck, Grumpig, and Latios. None of the Pokemon that innately have Simple are in the game, so that move is the only way to get it.

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Technically, Numel has Simple itself, though can't make very good use of it. I believe it's common for people to use Dondozo and Tastugiri with Costar Flamigo, as Tatsugiri gives +2 to all of Dondozo's stats with Commander. Pair it with Simple Beam and Order Up and Dondozo can get its stats raised high relatively quickly.
For one turn though without any outside help? Shell Smash, Belly Drum, or No Retreat are probably your best bet.

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i came up with medicham with the item roto boost, ability pure power, and acupressure but this is not a complete answer.

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This question if for Gen 9 so there are no Rotom Powers.

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I don't think it's possible to switch out Tatsugiri when commander is active.

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@sumwun You can switch a new Pokémon in for Tatsugiri if it faints, which is very hard to do, but not impossible. Wolfe Glick pioneered a somewhat gimmicky team based around this; Tatsu + Explosion Mon turn 1, use Endure + Explosion to drop down to 1 HP, then Tatsu gets poisoned by its Toxic Orb at the end of the turn. Switch in Dozo, then the poison kills Tatsu at the end of T2 and you’re free to switch in Flamigo.

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I think it's either just No Retreat Falinks, the Commander Toxic Orb Dondogiri tech, Belly Drum, or getting lucky with Ancient Power or Ominous Wind.