okay so. The main thing about this fight is the form and type changing.
Each form has its own set of moves, and as such its own set of types. Its been a while so I don't really remember it well so here's what I can remember.
Normal form is imo the trickiest. I got through it with either an endeavor or just bum rushing it with a strong super effective stab.
Defense form wants special attackers, it has stamina to boost defense. You want to end this asap since heavy slam only gets stronger the more you stay.
Attack form. Just have a sacrifice or use protect.
Speed. This is it. You win if you can deal this last bit of dmg. I once got here with nothing but a ghost type with ghost moves, so him turning to a normal type was a loss.
My team, if I remember correctly, was a staravia with endeavor quick attack focus sash, a bulky quagsire, abra, misdreavus, ponyta, and I think a crabrawler for fighting moves.