When he goes into Overlimit, get next to him and begin blocking. He should always use his mystic arte when in Overlimit and afterward he will hunch over to catch his breath. The more times he uses it in a battle, the longer he stays hunched over. I did not get this until the third time he used it, but just run up and use normal attacks or any arte when he is hunched. Eventually he will go down. Put your allies on defend if his health is getting low and you still haven't done it. Let him heal with his ice move for a bit before continuing.