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.
Defeating the Butcher
First, meet up with him in his room. Then, lead him to one of the walls that look like cage walls. Go on the other side of the wall and close the door behind you. Shoot him with a bow or use magic. Since he cannot open doors, he will just stand there. Just keep shooting him until he is defeated.
Find the Butcher. Open his door and run away from him. He should follow you. Lead him to a staircase and run around it, but stay close to it. He should follow you in circles around the it. Eventually, after about thirty seconds to a minute, he should get stuck in the staircase. From there, shoot him with magic or a bow. You should also be able to melee attack him. Note: He can get away from it if you go onto the side of the staircase that you need to be on if you were to enter it.
Open the door, cast Fire Wall immediately, then close door after the spell. The Butcher will be stupid and stand in the flames provided you stay at the doorsteps. He will die almost always before the flame goes out, even if spell is cast by a Warrior. Note: You must be fast and need to proceed a few levels down in the dungeon to learn Fire Wall.
Defeating Zhar The Mad
Walk towards a corner. He will follow and appear in front of you. Trap him in the corner. Hit him every time he appears, since he will be still trapped in the corner. If done correctly, you should not use any energy.
Duplicate items
Place an item in your belt. Take the item to be duplicated drop it on the ground. Walk away without leaving the screen, then click on the item again. As you pick the item up, click on an item in your belt immediately before taking the item on the ground. If timed correctly, the name of the item that was duplicated will appear in the box down below. It will have the same appearance as the item on the belt that was clicked on. Drop the item on the ground or place it in the back pack and it will re-appear correctly. However, do not create a second item of the same type and put both on the ground. If this is done, the duplicate item will be destroyed.
Duplicate large items
Put the item to be duplicated on the ground. Put a mana potion or another item on your belt. Walk a few steps away, then click on the item on the ground. Right when you pick it up, click on the item placed on your belt. Drop it on the ground. If timed correctly, it should turn into the item to be duplicated, and the original one will be in your inventory. This also works with money. Do not put both the duplicated item and the original item on the ground,or one will disappear. Note: This cheat will not work if the inventory window is open.
Defeating Diablo...