Did you do this recently? I boosted all the bosses damage too so damn! Yes please if you could outline your method, there is definitely an optimal methods but yeah ideally you just wont be able to do it at such a low level in future!
It was a matter of not letting them do damage. For e.g - as a mage , cast frost nova , cold blood , energy pot , nova , cold blood , energy pot etc . and slowly the boss died. As a warrior I don't recall what I did but there was some sort of a similar method. Maybe for future references a limitation of 2-3-4 energy pots can mitigate this and only be acceptable for onyxia etc :D