This fight seems impossible to me in its present state. Here's what I've witnessed:
1) Around 85%, starts summoning everyone on hitlist, stuns, and kills if they can't get out in time.
2) Mana drain begins around 75%
3) AOE knockback every 30 seconds or so, can be mitigated with levi
4) De-spawns/resets after around 8 minutes.
The timer is way to short given his HP. We had half the server (~30 toons) there last night with at least 10 donor pets, a group or wizards, and we couldn't beat the timer. Furthest we've gone is 49% when the timer expires, again with a large number of donor pets.
Regarding the summoning/killing, I don't know what actually kills the characters, it's not in my combat log. They appear to just die randomly. Some fall more slowly as if it were a DoT, but there is none on the buff bar.