Easy, peasy, lemon squeezy.
The only trick to this encounter is picking up the invisible named at the start. You will notice 4 alcoves where the named are, with Rwnzak out in front. The alcove with no named in front of it has an invisble Assassin waiting to pounce.
Have a Tank with good AE control pick up the initial encounter, with an off tank ready to grab the inivisible add (it becomes visible shortly after engaging.)
Burn down all of them in no particular order - although Rwnzak does have the most HP - and loot the chest.
The only thing I would suggest is getting range at the start as he does have a nasty arcane AE which is worth getting a timer on.
So in essence it's purely about DPS and dropping as many of the named as quickly as possible.