First, hire a mercenary from Griez. Enter Tal Rasha's chamber and cast a town portal for an emergency exit. The main trick for defeating Duriel is to bring lots of health potions and STAND and FIGHT. (You may use Blaze if you have the sorceress.) You will do less damage to Duriel with hit and run, with the exception of Blaze. You may die a few times, so keep your gold in your stash. Try to keep a Town Portal in the chamber to come back quickly. Also, Duriel's cold attack is a Demonic attack and not an elemental damage, so your cold resistance will be useless.