Ifrit Plumes and Eruptions with regards to casting mechanics.

[Frein]

You just have to get used to timing your heals like the DD have to learn not to spam weaponskills. Once Ifrit is glowing he will do weaponskills after 1-2 swipes so don't cast anything if he's already done one swipe. Ifrit won't attack the tank during any move where you have to run so as long as you keep the tank above 2500ish HP or so it is safe to let your tank eat a hit while you wait for a move to come out.

In my experience this isn't really viable in the last phase because the damage output on the tank is very high and you usually can't take any breaks from casting. Plumes shouldn't be an issue, the WHM has to move only if things are very seriously going wrong, but Eruptions are pretty much luck based when you play WHM because the tank doesn't ask permission to lose a bunch of HP in the last phase.