Your tank must be really well geared to survive Mountain Buster with 4 stacks of debuff...I'm not sure if any tank can survive it with 5 stacks (which is what killed your tank). But since that's the last one before that phase finished, looks like quite doable if the tank active Hallowed Ground for that one.

I wander how reliable is it to let the healers pulling the mini titan to the side (is that what your team did?) Which I think is the most difficult part because the tank will get 3rd and 4th Mountain Buster during that time...