sigma could shoot a blast from far away.. maybe from high above into the lava
i know.. shoot one into the surroundings of the arena to generate a lot of debris and rocks and then mess with the lava
note the lava is sloshy liquid.. what kind of attack spells are those? would they repel the lava effectively enough?
skin contact with the lava would be really harsh on kuja and the dragon where it touches

btw was it established that sigma isnt allowed to combine abilities from different forms?
im sure if sigma wanted to he could install some kind of device to allow flying and teleportation
and how about the fact that sigma has a shield and light sabre and a power scythe in other forms? does he have access to those?
could sigma somehow have all the weapons from all his forms available at once?