Duriel is tough as Hell.
I dump as many points as possible into Werebear before reaching him; up my cold resistance, and have something on me that cuts freeze duration... Make sure my Rogue hireling is buffed out (Hannah's my girl), pop a TP when I enter his lair, and then just duke it out, running back to town (or being sent there) to replenish, and back, until he goes down.
As for Diablo, I had a tougher time getting to him than I did once I was toe-to-toe with him. The Iron Maiden in the Chaos Sanctuary killed me more often than any boss did.
The Cold Resists and freeze duration cuts don't work with Duriel. Read the page about Durial found here:
http://classic.battle.net/diablo2exp/monsters/act2-duriel.shtml