I trained Shiva to level 16 both my Squall and Quistis were at level 14 and 13 respectevely.
I drawed cure from Blue Eye and a few sleep from Grat before meeting up with Quistis.
I use Squall to cast sleep on T-Rexaur and use Quistis to summon Shiva a few turns later the T-Texaur will die and i rarely get hit, plus i get dino bones which later i can refine into quake magic.
