Well I tried to PuG this last night and failed hard, mainly due to people not knowing the fight, so I thought I would summarise here.
Firstly if you dont have DBM get it! makes life easier!
This guy has three main abilities
Shroud of Darkness
This Heals him but main problem is when a player attacks him while Shroud of Darkness is up, the player will receive a debuff that decreases healing received by 20% and stacks 5 times (i.e. 5 x 20% = 100%) which means you arent going to receive any heals!
DPS STOP ATTACKING, DISPELL IF YOU CAN (or assign someone to do this if your lucky to have a dispeller in the group [magic debuff])
Void Spawns (ads)
Void Spawns will appear that throw all sorts of shadow damage around, these need to be nuked down by the person who has the DoT from the Third ability
A small DoT but it also allows them to attack the spawns.
I'm sure that with the right group (high dps and a dispeller) you should be able to heal through and get the achievement but it's safer to take the ads out.
In Summary this really isnt a DPS race, follow the strat and win win win
EDIT FOR DPS: I did this fight last night on my hunter, quick tip for DPS is to constantly be hitting TAB until you see target change then dps the shit out, they go down damn quick =)