I'm gonna interject here. No, Kuja doesn't "shoot" his magic at an enemy. It's not a projectile, it's a magic spell. Magic homes in on the mana energies present in and around all things. Hence the reason why a character can be blinded in a Final Fantasy game, yet their magic still hits without a problem. Kuja doesn't have to "aim" anything. The standard rule for magic is if the target is visible, even a little bit, he will be hit. If he's in another room, and is blocked off by an opaque wall, the caster is out of luck.

Oh, and I just caught this, mainly because I had forgotten entirely about the main platform in this arena. I assume that a lot of the action is taking place there. Well, transforming into a metal behemoth wouldn't necessarily be too hot an idea for Sigma considering that the whole place is erupting and that central platform is suspended by chains. Too much weight and the whole thing might go down.