Not much of a trick that I know of... I always had enough agro to go fight the fire sprites and still have agro afterwards. It could be that simply you didn't have enough lead over the healer prior to invul.

Healer healing a lot would do that, or you not building as much hate as I did (your weapon good? Using a good mix of Maim & Skull Sunder?). Were the little fire things blowing up? Not sprites, the smaller ones. If yes, that would force your healer to heal a lot.

As for building agro on an invul target generally speaking, Flash and Provoke (Gladiator cross-class abilities) are the only ones that I can think working. Just because they do no damage. Haven't really tested though.