To be fair, the Ishgardian forces directly involved in the battle with Vishap are horribly ineffective. They mostly just die in droves.

Just went through this a couple of days ago on an alt. It's stated that some of Ishgard's forces are on the city walls, manning cannons. Others, along with more adventurers, the other Scions, and the Crystal Braves, are scattered around Coerthas fighting other Dravanian forces. And Estinien was shown in a cutscene at the end, standing over some dead wyverns to explain his own absence from the main fight.