I wouldnt go as far as dive into lava. It wouldnt destroy is armor right away but it would weaken it greatly making spell deal even more destructive damage. There lot of place into the cavern is going high and it's not like Sigma can aim with the Lava. also, you just given me an idea. The Dragon can use his Psychokinesis to throw Lava at Sigma. The Twister (all), Shockwave (all) and Aerial Slash (all) could repel the lava making it fall down back on Sigma weakening his armor because of lava and receiving damage from the spell. Tidal Wave (all) could also be used to weaken the armor as passing from hot to much less hot to hot again is something destructive. So the lava would be playing for Kuja. I also doubt that the lava area is endlessly deep which would make it impossible to reach the higher part of the arena if it can even reach the half.

Edit:

Yeah, thats right i was talking about is huge form when lack of space not humanoid.