I second C Elegans on two points:
1. Duriel in normal is indeed the hardest bosses especially since the room is so freaking small.
2. Get a mercenary, give him/her the best weapon and armor you can afford, and let Duriel concentrate on your merc as you deliever damage from across the room. Raise as needed.
* The absolute best thing I did was that as soon as I entered the room I triggered a town portal, delivered damage, and then got out before I was killed. If your merc survived he/she will follow. (If not, go and raise from the dead) Go get healed and return. Make sure you have a town portal in your inventory before returning and make sure you have plenty of heal potions.