You are supposed to have the commander guy's skill active before both the big fattie and the boss mage start obliterating your team. Just turn it on once it is available and try to kill those two before it expires after 2 minutes. For the fattie, it helps to not have any melee on the boars until the end of it. It's mostly a ranged dps battle, except using price to block fatty and something else to block the mage.
And it's true that killing him before he moves is a possibility. My spica alone gets him to half before.