Just kill her.

I just did this very minute. However, I got my squad up to the lvl 17 area. 1st round I had my casters cast as high a level as possible of soul and element shield. My fighters (including my ranger) all drank potions of superman except my monk, who used a canned elemental. From then on my Bishop alternated between heal all and rest all. My mage uslessly cast concussion while the rest of my group went hand to hand. The potion of superman really increased my damage, I think that was the key. By round 3 Nessie was just another skeleton! I noticed two important things, first is that it looks as though bard instruments do NOT work under water (I really wanted that haste.) Also, I have a Samuri with the light sword and enchanted wascwhatever. A fairy ninja with the cane, NPC r2d2, a ranger with eagle eye, a bard with fang, and Vi with the dread spear. I had a lot of hits on Nessie and she never showed an effect. She must be immune to all weapon effects. Vi hit her 6 times with a 50% hex and she wasn't hexed, for example. I was assuming I would get an insta kill of a reload. I never figured that I would win a straight up battle. Good luck.