Kuja was surprisingly easy, though it helped that I had Zidane's Ultima Weapon, and Steiner's Climhazzard did over 9000 damage. (Plus Steiner entered Trance mid-fight, so even his basic attacks did 9999 damage)
Now it's on to Necron, who comes out of nowhere in terms of story but seems like a fitting representation of the game's themes. (Fear of death and what you choose to do with the limited life that you have)
Grand Cross definitely seems similar to Supernova. (A big flashy astral-themed super move that hits players with a bunch of status effects) Thankfully not as long, though it petrified Vivi, made Zidane berserk, made Steiner blind, and killed Garnet, so it took me a bit to recover from that.