I apply two basic rules of thumb when it comes to AoE healing:
In the situation where there is an AoE that hits the group...
Is there any more incoming damage to the raid in the next 5-10 seconds?
- Yes: Better Cure 3 (T13 Megaflares > Rage of Bahamut)
- Nope: Just Medica II and let the regen do it's job (T13 Megaflare > Flatten/Flame Breath x3)
In the PvP Scenario, I can't think of a reason to use Cure 3 at all aside from remote healing two melee getting their face beat in.
- Your group shouldn't be stacked together, that's just asking a trigger happy BLM to double flare your group while the WHM stuns you to oblivion.
