There are several 'Knot of Rust' items inside the prison. They deal some damage to any enemy targetted with them (which will help since you don't have any weapons for that fight). I suggest getting all of them, and using them to take care of the boss' henchman, and then concentrate on him alone.

General RPG standard, but if your levels are at the point where the characters that join your party or above, then you're okay in that area.