HOW TO KILL DURIEL (in Diablo 2 LOD v1.10)
{this refers to the Barbarian} I have had trouble beating Duriel as well. When I first got to him I had thoroughly explored act 2 and he killed me immediately. I was at level 22. I knew that he was susceptable to cold attacks and had ridiculously high endurance as well as the ability to freeze you and make you dizzy. He also likes to block teleportation gates making it impossible for you to escape.
PART 1 (EQUIPMENT and STATS)
I concluded that I needed to be able to do more damage and last longer (saphires for my weapons and more vitality). I went back to the first act and played through all of the levels to get the gems and used my horadic cube to upgrade them as best I could. That didn't help me gain alot of experience though. (HAD I KNOWN AHEAD OF TIME THAT I WOULD BE NEEDING SAPHIRES LATER ON I WOULD HAVE HAD PLENTY AND THIS STEP COULD HAVE BEEN SKIPPED).
To gain experience I went back to the seven tombs and searched through them thorougly as though I had never been in them before (entering every room and killing every character). After running through the tombs 2 1/2 times I was up to level 25 and my merc was up to 22. I PUT ALL OF MY NEW STAT POINTS INTO VITALITY FOR LEVELS 23 through 25.
My stats were as follows: Str-77 (upgraded to 90 using charms), Dex-45, Vit-69, and Enr-14. My relevent skill for this ordeal were Level 8 for Double Swing and Level 3 or 4 for Mace mastery (passive). My merc stayed on a high level because I continually brought him back to life.
I was armed with two Grand Sceptor Maces. One had three sockets and in each socket was Blue Saphire. The other was a Magical Grand Sceptor and it had +75 attack rating, +19 Maximum one handed damage, +25 Maximum Damage (I got it the second time through the seven tombs). My armor for me and my merc were pretty much fully maxed out (for the Act 2 stage). I used no special armor pieces (sometimes you find pieces in one act that don't normally appear until the next). I even had flawed skulls in my helmet and armor sockets to help me recover damage.
The saphires I used were of the Square quality. For my mercenary they were the tear drop quality. My skulls were of the flawed skull and cracked skull quality. I USED THE HORADIC CUBE TO TRANSMUTE LOW QUALITY GEMS INTO HIGHER ONES. Though I came across about one of my Square Saphires and two Tear Drop Saphires, it probably took no less than 15 to 21 chipped saphires over all just to equip my Mace alone. Saphires do cold damage so they are good to fight D with.
PART 2 (METHOD)
Once I had these levels and equipment this is the strategy I used:
I enter the tomb Duriel is in and fight off all the hords previous to D's room. I use my skulls in my armor to heal (by waiting several minutes) rather than my rejuvenation potions. Then:
a) I take a thaw potion for me and my merc. I enter the room of Duriel and immediately cast a portal and run away from it to prevent Duriel from blocking my entry later.
b) On the other side of the room I attack him with double swing and as soon as my merc is dead I run in the opposite direction from the Portal to lead Duriel away from it. Then I double back into the portal. All this time I am continually using health potions to keep myself from dying. (I customized the controls so 'Space Bar'=Portal and 'B Key'=Take a Rejuvination potions).
c) I go back to Act 1 to bring my merc back to life. I get previously stored rejuvenation potions out of my stash and replace my old. I take a thaw potion for me and my merc. I go back into Duriels layer and repeat STEP A, B, and C as shown above. Rinse and repeat.
After doing this 3 times he died. It was not even hard. I was amazed considering how earlier I was only slightly weaker and less well equipped and he was whiping me out instantly. You just have to have your stats JUST RIGHT.
Although this was done with the Barbarian I imagine the same strategy would work with other characters as well (with a little tinkering).