That being said, i'd put this raid encounter at a high tier 6 at the *very least* simply due to the amount of mana it requires to keep everyone alive. Without spoiling anything here, i'm sure those who have done it/made it know what happens, and I can't think of any other encounter in tier 6 where healers have to have such a large mana pool.
I find this quite odd, because this is like the only fight that I can think of where I find myself bored, and loading Ymir himself up with like 3 DoTs (I am a druid). I can see where maybe having a lack of dps can really be detrimental to healer mana pools in this fight moreso than others. I am at a loss though.