Diablo is easy even in normal. Just get decrip and a few points in clay golem and like +3 clay golem wand and + clay golem necro shield on switch. Keep Diablo cursed for the whole duration of the fight. If you're lucky you have found Amn rune before and made Strenght runeword(Amn+Tir) for your mercenary before the fight in something like a pole axe.

This. I killed diablo not that long.
Decrip all the time + resummon stone golem as tank. Keep him with your skeletons far from you.

Stone golem?

Pretty sure he means clay golem. My necro I put 5 points into golem mastery and 2 into clay. Then respec to purely skele after I hit nm act 3