KH2FM+ CM - Hydra
Disclaimer: Kingdom Hearts 2 Final Mix + belongs to Square Enix and Disney.
I considered this to be an awkward fight. Surprised myself at winning first try.
Immediately jump over the tail twice. Dodge the black things that come out of the ground, run away from static charges in the air. Combo him a bit, Vanquish RC, Urninator RC, beat up and vanquish the 3 heads.
Run normally to avoid trail of fire things. Same tactic for previous two attacks, I don't think he sweeps his tail anymore, not certain. Avoid center field when not attacking him, they bite down on it.
Combo the heads when a solitary one ducks, run like hell when they all lift up. Do the RC with Pegasus for a lot of damage, land one final hit after critical HP for end, then Vanquish. Defeated.