Kuja would just thrash him with Thundaga, Flare Star, and Ultima and Sigma's toast. Sigma's a frigging robot, right, so he's REALLY susceptible to lightning. And yes, Kuja flies around. As Trance Kuja he flies until you defeat him before Necron, and that's pretty long (or was it it with Garland?!). And he has his awesome Silver Dragon. He's NEVER without his trusty steed!

Life is short.
Let's all grab a BEER~~~!!!