Nessie isn't too hard, even for sub-15's parties once you know what she's capable of doing. [img]smile.gif[/img] I took her out with a level 13-14 party, and one Canned Elemental (summoned two but the freak lizard snapped him in two in one round
) by sheer brawn.
None of her attacks are completely brutal, except maybe Concussion, and her melee is fairly weak. About the only thing you want to cross your fingers on is she won't cast Body of Stone. If she gets one off, it makes the battle about 10x longer.
Personally, I think the tactic of hiding from a distance and hitting her with 1000+ arrows, stones, and bolts is a pain. Nessie can still hit the party with her magic from anywhere in or around that den of hers, and they hurt more than her melee even with max resistances. By drawing up close to Nessie, she'll go to melee 9 times out of 10.