-Lure out the lesser monsters a couple at a time and kill them. This reduces the number of enemies that come after you. When Diablo pursues you, get your back against a wall. He will not be able to knock you back and you can keep hitting him.
-Go to the witch and get Firewall, Flamewave, Fireball, Mana Shield and Stone Curse. Duplicate the items as many times as possible. Read all of the books. If playing as the Wizard you can read more books. Purchase as much Mana as you can get in the inventory and the hotkeys. Return to Diablo's level and activate Mana Shield. Kill off the littler monsters with Fireball. When you get to the main part where Diablo is located, cast Firewall as many times as possible. Refill on Mana, then cast Flame Wave as many times as possible. Refill Mana again. When Diablo gets into view, run in to one of the smaller rooms on the side. When he enters after you, Phase or Teleport out and set Firewall in the room. When he exits the room, keep about a hands length away from the door and you should be able to kill him with fourteen to nineteen Fireball spells. While he is chasing you, fill the hotkeys with Mana and you should be fine. If you want to stay away from his spell, just keep moving.
-As a sorcerer, get to around level 25 and make sure you have a high level Holy Bolt (10 or higher). Finally, make sure you have plenty of Mana. Go to where Diablo is located, take out the enemies around him, then blast him with Holy Bolt. Since Holy Bolt is at such a high level, it will shoot very quickly and Diablo will not be able to move or get out of the way.
-To not have to kill all of Diablo's minions first, use your automap (press [Tab]) and line your character and the small orange arrow up so that they are exactly together. After that, stand in the opening of the third room of levers that is facing Diablo's chamber. Line yourself up with the opening, using the map, to his room without leaving that doorway. This works better with a Rogue and a very strong bow, but any sort of projectile spell will also work. Shoot directly ahead until he smacks you with his fire spell. Then, teleport frequently unless your character is very strong. Alternately, you can get your back against the wall so that he cannot knock your back.