It depends on which chapter it is, I imagine it is in the third one, right? But regardless, the mission is not to kill a Jotun with regroup damage, you have to have a captain with this ability on your team when you go to fight, then you have to go to the underground caverns and fight a Jotun, then you choose which Captain Britt you will go with, and you choose which Jotun you will fight, I don't remember the Jotun options, but there is one that does more damage and one that restores more health, I recommend going against the one that does more damage since you will have someone restoring your health