I think Assassins are probably the most powerful character in Diablo II. Druids get pretty nasty too.
To take out Duriel, I have the right-click (f9-f12) buttons set up with f9 having the throwing blades (shuriken) and f12 the double image (or is it the mirror image). Run around till Duriel attacks the double image, then you can stand back and attack him with the throwing blades. Keep one hand on the f9-f12 buttons, and when he kills your image, quickly throw up another one and then continue with the shuriken attack.
It's also a good idea to hire a henchman from Lut Gholen, and make sure he regenerates health for you.
I you'd like help getting past Duriel, I'm good for a game just about anytime. :)