To defeat bastion, I'm pretty sure you have to be Wolverine, and since Wolverine is my best character, here are some pretty helpful moves:
Animal Instinct, level 1, back and AB at the same time, does a barrage of punches and cuts
Precise cut, level 2, AB at the same time, slices the opponent 20 times into the air
This next move I don't know what it is called, but you jump up into the air and press AB at the same time, its level 2, and it is some spiral cut attack
Berserker Fury, level 3, forward and AB at the same time, advanced version of Animal Instinct.
Regeneration, level 4, XY at the same time, regenerates about half of your health.