You should have enough willpower so that the small rats doesent wound anyone when they are struck, and forgrimm should be tanking the boss and he should have as much willpower as possible so he (almost) never gets the doublewound from her.
Also, I didnt think about the fact that you can lure some of the smaller rats first, thats a REALLY good strategy, I loaded my game there and saw it now. Only problem was that I was continuosly using a summon which ran right at them:speech:!
If you have an elf in the party he/she can cast
meek you be and it will disable some of the smaller rats for 1 minute, I would advice to not overuse that one in the beginning since then you might need to fight all the small rats at the same time
having the clarum purum spell at a reasonably high level can remove poisons from one party member and make them immune against poison for a couple of rounds.
Obviously using a summon is really good
except right in the beginning if you are going to pull some small rats first before the boss!
*edit* oh yeah,
obviously it's good to have some healing potions..