Wakka's celestial powered up is one of the key items man, I mean Break Damage limit + attack reels + a nice amount of strength = 12 x 99.999 damage. against most monsters and bosses.. xcept for the dark aeons that's bout half as much.
Shiva is faster than Ixion I believe, but you have to fight Ixion twice (he's way easier the second time) and the first time you fight him he uses sleep attacks if I remember correctly
