You could lure him out, far away from spawn. If you initiate fight and don't finish a boss, it's hp will return to 100% but aside from that and if away - he will wait for u.

if that's some nasty bug with that boss doing you instant before you get any chance to run few meters so u can try to escape, drop me an invite, I'm online for an hour or 2 more and can simply help u with that fight