Ok at the third boss of Utgarde Pinnacle (Skadi the Ruthles). When he is in Phase one he will cast Frostfire on the ground (~3k dmg a second). Now i found a way to always avoid getting damage from it. On the right side next to the harpoons is a snowy hill. if you stand on it you never get damage from the frostfire. See my screenshot: if you stand where the arrow indicated you never get damage. This is perticulary handy if you are ranged since you can just keep shooting the mobs.
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