Just beat it with Human Bloodmage. Used the "attraction" preparation to know instantly whether ENDISWAL had spawned and scummed until I got it. I also got a rather expansive subdungeon (ying/yang) that gave me enough mana to make the upper level porous, which made the ensuing boss battle a breeze.
@Wargizmo: I'm not
having problems "beating" the boss. I'm getting to level 7 easily with poison antics and getting 1-2 levelups during the boss fight with the various monsters I've left behind, plus all my potions still intact and the map littered with bloodstains. Winning a straight fight at this point is fairly easy. What I'm complaining about are those damn brambles. I'd have beaten him several times over by now if not for getting locked down by brambles.
For preparations, make sure you take the smuggler's den
Considering the brambles will often cut you off from retiring, I wouldn't dare go with anything other than insurance here when packing a relatively rare item like the venom sword (plus this late at night I have a nasty habit of mis-clicking)