Before reaching the boss, have a healer with moonbarrier stand right in the middle of the path to reach the boss place (without entering it) walk slowly, and when the tree on the left sends you a flying snowball, inmediately put a moonbarrier right in front of you and retreat, if done correctly, the boss will come right at you, have him come at you and walk down the narrow path, when he's completely out of his area, procceed into the boss area. He'll be out and wont be able to come in, allowing your ranger attackers to kill him easily (or your ranged attacks) (just watch out for the trees though)
When the cobolts says something about releasing the reaver (and that you broke the box containing him) you can block him by having somebody stand right in front of the reaver. Giving your party enough time to clean the mob before the boss appears (or the reaver gets bigger which is the same thing)
When fighting the boss. Have 1 person stand in the way of the boss right after the boss appears sign shows. DON'T attack the boss with anything. And the boss won't attack back.
In the meanwhile the party can kill the mob (catapulters which are very annoying and stuff) works most of the time.