-Run in circles so that he cannot hit you while another player attacks him in the back.
-Get your character to about level 10. Go inside his tomb and take out all the minions that you see. There will be multiple rooms all with minions in them. Before every room, go to the very edge of it so you are not in the room, but can see inside. If you see an archer to the left and another to the right in a very big room, save the game, Charge in and make sure you have the Fire Wall spell and at least two Full Mana potions. Cast Fire Wall as many times as you can in the center of the flood of monsters that attack you. Make sure you do not allow any skeleton minions to get behind you (archers do not matter). Eventually, you will see a massive skeleton come after you. This is King Leoric. Wait until he is within striking distance of you, then run away as fast as you can. Make sure he is directly behind you. Lead him to the room that you would need to be in to exit his tomb. If there are any enemies here besides him, lead them all back into the large room and kill them. If Leoric is left alone in the same room as any of his dead minions, he will resurrect them. After Leoric is led into the room that you would need to be in if you were to exit the tomb, do so if necessary and cast a Town Portal. Buy five more Town Portals and completely heal and fill your belt with health potions. Return to Leoric's tomb, but do not enter it. Get directly outside of it and save the game. After that, enter it and he will be waiting for you. Hack away at him until he has almost killed you. Once again, climb up the stairs and refill on health and mana. If needed, go back to town and buy more items. Leoric will not refill on health, allowing you to repeat this until he is defeated.