He would destroy himself. I might be destroyed. But i most likley survive by destroying falling rock. Sigma would most likely be destroyed by his own attack. He cannot sustain himself with nothing and while he destroy the cavern he cannot protect himself since he is already busy. So he woud be draged by the rock falling to the bottom then destroyed.

His dog projectile are lame. One has to follow the wall and the other as a shitty range. He can jump from wall to wall but he need to stop a few sec once a while. This is why with no ground he is doomed.

Sigma cannot possibly be covering a big area with his body. Bigger his body is less movement he would have. This arena aint a cube. It's a cylinder.