Diablo's is so pathetically easy if you do jsut one thing to him. cast BLIND on him. it may take a few tries but once he is blinded he will rarely be able to use his physical attacks on you, and since everything else he uses CANNOT kill you, you can beat him regardless of your level.

"and no PG diablos is immune to blind so therefore that wont work"

that is entirely untrue, i blind him evertime i fight him and it always works, it just takes a few tries.

EDIT: Do not double post. Use the edit/delete button to add more to your post. -Murder