Most important part to this raid is rezzing. Make all your raid have hotkeys for /corpsedrag
The moment someone dies, their groupmates should be using that hotkey. The NPC's that drag corpses to the pools can't drag corpses if another player is corpsedragging it.
Don't use shaman or druid calls, always use real rezzes.
Have a shroud team of about 6-7, place your 4 most competent melee in charge of permanent shrouds. Stick those 4 in a group with 2 dedicated healers, we use druids.
During the living phase just have those 4 people on shroud duty, during the undead phase have everyone in shroud crew rotate in, since undead phase is harsh and has to die fast.
The living sacrifices can be single target buffed and healed, Marnek attempts to kill them during undead phase, you can't let them die or he powers up. It's winnable by letting 1, sometimes even 2 die but impossible basically if all 3 die.
It's also important to note that this is not a burn fight, slow and steady DPS will win it. If you burn, you'll overrun your raid with Wraiths and your shroud team won't be able to keep up.