What classes are you using ? DPS as range ? I assume you have at least 130 frost resist. Tank really needs to have 30k hp unbuffed. Paladin tank can may be having at little less since you got that "cheat death back to 30% full OPness".
Two things make this encounter hard. Tank damage and party damage. You are controlling the tank. So there are some ways to avoid getting 1 shotted. Your tank should at least take the beating from the red beam (coming from one of your dps). Heal through that. When he does a knock back, (this is by the way to help you escape, to start kitting), he will enrage and become larger. Find a icy patch on the ground, and kite him. If no icy patch around and he got near you, this is when you blow your defensive CD. His forceful smash can easily hit the tank for 15k-19k. My pathetic dk tank was getting 1 shotted until I got 32k hp unbuffed.
Party damage. Once in a while Rimfang will mark a person in your group and shoot a frost bolt at him after. He will be deep freezed, so is anybody around him. This is not too bad. This person will be marked on your healing UI. Heal him for that. What killed my group is when I played range dps (shamans) is this, standing in the icy patches on the ground, (same ones you use to kite the boss), the person who stands in it will take constant damage and will be snared. I find it kinda hard to heal him and the tank at the same time. But sometimes, you get lucky. Nobody will be standing in it through out the entire fight. So no moving could work. If you do get this, I suggest to move that character.
Some other stuff that I noticed, he will at least enrage twice maybe 3 times depending on how fast you can kill him. So kite him once and use defensive CD for the other. For paladin tank, this is not so bad. Cheat death once. Divine Protection the second.
Rimfang does the mark on your party one by one. He doesn't really mark the same person twice. (i don't know for sure, but it seems that way for me).
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