A tip:

Start the encounter with all your team at the bottom of the steps and pull one of the skellies to begin. This way when Garfrost runs to an anvil, he LoSes your group and the debuff drops. No need to run for boulders. Picked up [Doesn't Go to Eleven] this way today too.

Key things for me is to keep everyone on follow and refollow often to chain heal, EXCEPT for phase 2 where you want to spread out to keep the extra frost dot from the wave down to a minimum. I blow heroism on phase 3 just to end it fast. I get 3-4 rocks at most the entire fight this way.