Have you thought about him starting out in his normal form, then if beaten in round2 again grows to his giant size with full power. Now thats a right propper boss battle.
-The Horsemen: Intro of them kneeling, but thats it. He normaly fights 1 to 1.
-Shield is a bit off, PM me, I'll see what I can do for it.
-A clobering move like Sentinal's 'Rocket Punch' would be great as well.
-You can make him grow/shrink via code, so I wouldn't worry about that much.
-You'll need voice clips, these should help-
"Beast, how maney peoples have dreamed of my end"
"As your world passes away, we shale forge a new one in fire and blood"
"I am as far beyond mutints as they are beyond you"
"Am I doomed to struggle with such, FILTH untill the end ofr time?"
"I am the rocks of the Eternal shore, crash against me and be broken!"
Gold, frigging gold!