Quaggoth blood plus bishop of air. Spell evasion works on blasts of gas. Can learn even though you're immune when you don't evade.
Now that the blood is not immunity, this is riskier, but probably still works.
I recall taking scouts there so they could perfect it risk free relatively quickly (since that guy's fire rate was insanely high), I just tanked him and spam healed.

A certain area explore is a pretty good place to do it as well, since there are a number of invoker npcs.

I found that by ranking on mobs that did area spells it perfected pretty easily. The best part was you don't even have to be the tank, since you're only avoiding area spells. So face tanking builds can still master it relatively easily and effectively.