Struggled with a lv7 party a bit, then decided to ditch him and just sneak past.
On the way back I had to use Shadowing Beyond to avoid being one-shot, and while I was at it, I hit the bastard with Gouging Strike as a kind of a parting gift.
For some reason, I was never out of combat, though I already ran all the way to the edge of a map. Turns out, this thing literally never ends: the poor bastard just bled pure damage until death and I just had to hide and wait. I guess it's a suitable way for an assassin to kill the big game after all.
TLDR: Gouging Strike then wait.