Trio question.

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If you're a nec/enc duo looking for a third, any priest would be great.

-Cleric CHing charmed pets is very effective, as well as extra HP/heals for charm breaks.

-Druid can bring snares/regen/DS/charm break healing/etc, and sometimes can add an additional charm DPS

-Shaman can of course do all the stat buffs, regen that necro, heal for charm breaks, add malo line of spells to stack with the enc's tash for charm, and can take over slows to save the ench more mana for charming/CC

-Mage also comes to mind, if you're already in a pet heavy group, why not add more pet?!

If you wanted a more permanent tank, consider SK or Paladin, between necro heals + Enc slows +C, Healing should be manageable.