You always either have to mezz healers or kill them first. I like to get them out of the way, then unthaw the other stuff. If you see one casting to the side, go ahead and mezz it.
____________________________
Trolls just don't seem to minuet and pavane with grace and vigor. anymore.
She is not just any healer, because she casts Complete Heal and nukes like crazy.. I have had tons of tanks stunning her while she is casting, and it very rarely disrupts her concentration.