Achievement - Hadranox Denied

Hello, i've got one question, how does this achievement work? We tryed to kill him right away after he moved up, but the little adds keep spawning non stop and he's restoring his health. After 10mins of fighting we just resetted him.

Is there a special tactic to get this one?

you have to pull him all the way to the back so the ads that come down dont aggro on him...the ones you pull with him will eventually die and you can kill him.

Thank you very much, gonna try this tomorrow. c: