He takes no damage from physical attacks and is magic resistant. It is possible to defeat him using critical hits with poison damage; however, doing so will incur a reputation penalty and will not enable forced entry into Candlekeep (as the gate which he guards will remain closed).
"Hold, travelers! Before you will be allowed entrance, you must donate a tome of great value to our libraries."
- 1:-We don't have a book, but I used to live here.
- I am sorry, friend. I know that you are the child of Gorion and have dwelt here all your life, but I cannot exempt anyone from the sanctions of the most high Ulraunt.