Well in this battle i would say the only chance Sigma would have in his Kaiser Sigma body. As it seem to have mobility. Limited but mobility none the less and lot of projectile to offer a better control of the arena compare to the other form.

Kuja is sure a caster, but he is a pure one.(No Melee/Caster hybrid stuff) So his casting sure does take concentration and time but it would be very small to instant(Depending of spell of course.) with a small recovery time depending. Kuja physical is good. I wouldnt go as far as say great or anything but it is good. His armor that he has equiped is based on mobility and got some resistance. The dragon could be an easier target to take down but he is just as resistant (hp) as Kuja. So it would be possible to throw a Curaga once a while to keep him alive. The Nova Dragon would mostly be attacking and counterattacking while being the main target. Kuja would like i said throw a heal once a while and well use other spell during the rest of the time. If Sigma is in his Kaiser body. Kuja could destroy all the chain holding the plateform making him fall at the bottom. (Is there some place on the bottom without lava?) If no well game over. If yes, Sigma would probably use a little of his time to reach that or one of those area. It would limit his accuracy at because of high range giving lot of time to Kuja and Nova Dragon to dodge attack. From the top. After a while Kuja would Trance! Then would come Ultima that would extremly damage the whole structure and leaving probably nothing but a few remain of Sigma.