Lure out the lesser monsters a few at a time and kill them. This reduces the number of enemies that come after you. When Diablo comes after 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. Buy 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 smaller 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, remain about a hands length from the door and you should be able to kill him with fourteen to nineteen Fireball spells. When 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 about 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, kill 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.