*Spoilers* (I guess?):

A1 - When bosses land together on one side of the floor.
A3 - When he casts Cascade (I've had DF parties that didn't manage to kill all adds during "tornado fountain" and ended up with a vulnerability stack, and would die to Cascade without mitigation.
A3 - When he transforms into a hand and people stack in the middle to mitigate damage, it helps and the other healer can heal that little damage you take.
A4 - When each leg dies.

Ravana - Right before he finishes his ultimate cast.
Bismarck - Right before he appears in front of you to do its ultimate attack.

That's with nocturnal sect pretty much. With diurnal sect I only use it on Ravana after his ultimate, 'cuz I can stay up a bit longer and people top their HP while landing on the floor and saves MP from AoE healing before the push to wall.
So far it's fine, the only complain I would actually agree is the mobility vs mitigation unbalanced. If they're gonna make us unable to move, mitigation should be at least 5% higher.