Of course you guys find it a joke knowing all the strats already (or have someone telling you what to do). The first week was thoroughly enjoyable as my guild went in guns blazing and blindfolded. The lack of trash was great.

Our kill priority is always the Disc Priest > Shaman > Druid > Paladin for the healing npcs.

You will need a dedicated dispel/purge on the first kill target, and another dedicated CC on the other healer, purge everything on the kill target. So in your case, focus on the Priest and have your Warlock focus on the resto: banish, death coil, fear, Curse of Tongues. By then the priest should be dead. What you kill next depends on your raid health.

Remember, doing DPS isn't the issue in this fight, the key is control. If you have warriors in your raid telling them go Prot for multiple charge stuns and shield bash silences might be better. Mortal Strike debuff is nice but not really necessary. The Enhance Shaman and Rogue are harmless if they can't reach you, use roots, snares, stuns.

In my experience the Mage don't really do fatal damage, more like being an annoyance with the poly and frostbolt slows.