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).
If you return to him immediately after Gorion is killed, are nice to him and beg for assistance, you can get 100 from him for free.
"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.