Damnit, I thought I was done with the damn game when I beat him, then he turns into this huge thing with a full health bar
Ahaha, I thought that was what you were struggling with. If it helps, the first one isn't so difficult to kill with a projectile character. Spam beam supers like crazy. The second one moves around a lot, and is more frustrating. At least to me...
Banned, use Mega Man and War Machine to beat Onslaught. MM is small enough to still shoot his fireballs while crouched during some of OS's first form beam attacks.