The Possessers in Achenai Crypts will cast the MC debuff (not dispellable iirc) which persists for either 1 minute or until the MCd char has his HP reduced to 50%, whichever comes first. Targets #1 on threat list.
They have less then 4000 HP (1 volley from 3 or 4 DPS). They are only summoned after engaging a Draenie for approximately 2-3 seconds. Nuke down your first target (should choose a caster) while checking for spawned Possessers, and then take down any that spawn.

The Timeless Controllers in Sethekk Halls will drop a MC totem after approximately 10 seconds of being in combat. They should always be your first target. Targets random person on threat list.

All that said, pulls in Sethekk Halls are MUCH easier then Achenai Crypts, and the first boss in Achenai is almost impossible at appropriate level (some have been able to kill it with 70s on normal while 5-boxing). Just skip AC and never look back.

Lokked