Instructions
Normal Difficulty
Prepare your Necromancer with an army of skeletons, a golem and a hireling purchased from Act I. Additionally, you will want to purchase the "Bone Spear" ability for direct damage against Duriel. Split your build between the "Summoning" and "Poison and Bone" trees, focusing on "Raise Skeleton" and "Bone Spear" in particular. Equip your Rogue hireling with a bow that adds cold damage.
Enter Duriel's Lair with your minions in tow and immediately open a town portal. You will use this portal to escape if you are near death or if your strategy fails.
Stand your ground against Duriel. Though keeping your distance would seem effective, Duriel uses a devastating charge attack that only targets you if you run away. There is no way to avoid this attack other than standing next to him and taking hits.
Immediately attack Duriel with the "Bone Spear" spell. Continue casting the spell until Duriel dies or until your golem dies. If the latter happens, resummon your golem immediately and resume casting "Bone Spear."
Use a health potion every time Duriel strikes you and a mana potion every time your mana drops below half. Duriel can kill you in the blink of an eye, so don't take chances.
Escape through the town portal if your rogue and too many of your skeletons fall. You can always resurrect your hireling, resummon your army and try again. Remember to recast town portal after you resume the battle with Duriel.
Nightmare and Hell Difficulty
Set up your Necromancer with high-level "Fire Golem" and "Bone Spirit" spells. Bring a high-level and well-geared hireling for additional support damage, though keep in mind that this minion will die quickly to Duriel.
Open a town portal instantly upon arriving in Duriel's Lair.
Attack Duriel with the Fire Golem in the lead. Run away if Duriel targets or charges you. He will usually switch back to the Fire Golem. If not, you may die; try again and hope Duriel chooses to attack your minion instead of you.
Recast your Fire Golem every time it dies. Though the Fire Golem can take quite a bit of damage, Duriel is extremely powerful and will crush it after only a handful of hits.
Cast "Bone Spirit" repeatedly until Duriel is dead.