You will find the YAT-99 in the wedding portion of Bevelle, as well as after escaping from Via Purifico. When you encounter these, they are sitting in a second row, behind other enemies. While back there, melee attacks will be unable to reach them. Upon defeating all of the enemies in front of them, your party will move forward and you can attack normally. Their attacks in the back row consist of either attacking a single character or attacking the entire party, but they can only do single target attacks when they are in the front row.