Just checking, but you ARE having the tank taking the tolerance go behind one of the pillars to interrupt the cast via LoS, right? We do that and we get 2-3 stacks usually, sometimes 4-5 but thats it.
Good healers and the right defensive cooldowns, and it's not necessary to LOS. It may be helpful, but it's not strictly necessary.
I think the cast time needs to be increased on tolerance so the active tank can line of sight the boss. even with cooldowns I have yet to see a tank manage to survive it.
so be careful with it
It's doable. Granted the tanks we run are usually full 61s/partial 63s, and at least one or two of our healers tend to be up there, too, but it's easily survivable without LOSing if you save the right defensive cooldowns for it.