Changes

Diablo has by far the highest hit points, AC, and damage in the game (and they're raised still further on Hellfire), so prepare for a long fight. As with any tough fight, it's wise to stock up on potions in advance, and to make sure your equipment is fully-repaired. Drop extra potions on the ground if you think you'll need them, since it's faster to pick them up than try to click and drag them from your inventory to your belt mid-fight. The key to beating Diablo is the same as with most tough melee bosses; make sure you've got plenty of to/hit and a fast swing. Hitting him with your swings is more important than doing a bit more damage, so if you must choose damage vs. to/hit, take the to/hit. Being able to block his attacks (both his melee and his Apocalypse spell can be blocked) helps, as does doing sufficient damage to rock him.