Kudos to anyone who actually manages to kill that rat ...
I had my kensai, thief and shapeshifter-druid in Greater Werewolf form all beat on him after destroying Gaxx and *couldnt* kill it.
That thing is thougher than Gaxx himself, doesnt take more than 4-7 damage a hit and seems to have infinite HP
All im gonna say is make sure you have some basic protections when first entering the building ( Protection from magic -lvl 6- f.e. ), a trigger with 3 skull-traps would complement that nicely.
As for Gaxx himself,.. be prepared and make sure you have some Spell Immunties -lvl 5- memorized ( as much as possible ). Dont wanna spoil too much more, more fun too find out yourself