Duriel is the first real challenge for many characters. Here are some hints:
1. First of all, get a good shield. I don´t care if your fancy fledgling Frenzy barb looks like a wuss with it, or exactly how much your sorc loves her shiny new Leaf staff, get a shield!
Shop Drognan for a while to get a Bone Shield of Deflecting. It requires only 30 Strength to use and has a base blocking of 60% (or more, depending on class). Even is you have a no-DEX build, there will still be some blocking to be had from this shield at your character level.
2. Second, a good weapon is a great help. I recommend the runeword Steel (Tir + El) in a Flail. You can buy 2-socketed Flails from Fara. Good damage, reach and speed, and the Open Wounds is great.
A Sorceress needs no weapon though, she can just spam Static Field...
If you have an act 2 mecenary, I recommend you use some of your chipped diamonds to cube him a Savage Poleaxe/Halberd. Helps quite a bit with the damage (as long as he´s alive).
3. Get at least 35% cold resist on the gear, then drink a large pile of Thawing potions (buy from Lysander). This adds 50 to your cold resist and raises the max. to 85. Each potions gives this effect for half a minute and the duration stacks so you can drink 10 of them and it will last for 5 minutes. Do this for your mercenary as well.
Duriel´s Jab attacks and normal attacks will now do much less damage and you will block some of them.
Now you have the means to kill him, and a decent defense, but you need more. Which leads us to:
4. Potions. You did save up all those small rejuvenation potions up to this point, didn´t you? 3 of those can be cubed to a full rejuve potion. I always keep at least one column of full rejuves at all times. The full rejuves are for emergencies and to save your merc if you really want him/her to help.
Lysander´s puny Light Healing potions won´t do much good in a hectic fight with Duriel. I always save up a bunch of greater healing potions from the seven tombs. Regular (mid-sized) healing potions are also quite good.
Get a belt with at least 12 potion slots.
Drink healing potions often, even when you don´t get hit much. It´s very good to have one 'working in the background' the whole time. You´ll heal between hits and it´s a good way to get less dependant on your (probably few) rejuve potions. This goes for the merc as well.
Last but not least, take the fight in several rounds. You can´t run away from Duriel but you can take a Town Portal, get healed, resurrect merc, buy more potions, etc., then go back and pound the stupid slug some more. It´s a good plan to cast the TP early in the fight, then get out when you get low on potions.
Good Luck!