Well this was our 1st kill for the achievement and as you can notice it was a rough, mainly zerg kill.
Found out some hints, though, on how to do this properly and leave the RNG factor a bit aside.
First of all keep in mind that 2 things will wipe you:
When the boss comes up the slope tag it with a low threat ability. Faerie fire or any other rank 1 spell will do.
Let the boss start killing adds and observe where and when he casts the asid cloud.
When the cloud is about to despawn (~1m after casting) pull the boss taunting (unless it already attacks you).
Once the poison cloud despawns the spiders that come down the stairs will continue down the slope and wont aggro you.
Get him back away from the stairs and start killing its adds. Whenever you see Acid Cloud in the group kite the boss ASAP away from it, because the combo Cloud+Poison on the spider adds will kill them and heal the boss.
If your aoe is poor try ignoring all adds and go directly for boss.
Note1: Poison Cleansing Totem is always helpfull.
Note2: Heroism helps but we did the achievement without hero.
Note2: A disease cleanser is also welcome, as the adds will put a +50% dmg taken debuff on your tank if he aggroes them.
For any extra questions you can find me as Alie @ Chromaggus.
[AH - Achievement Whores] and
[Alie n Friends]