Punching Diablo?

Easily done, unequip your weapons, get close to diablo and quickly swap your primary attack ability, so it enters in cooldown and you'll be able to punch him when attacking, by doing this, you'll instantly get the achievement, no matter if you die after that. i just got it now: 09:40a.m 16/05/2012 (-3:00 GMT)

Similar to The Hero's New Clothes achievement, you literally have to unequip your weapon(s) and attack Diablo with your fists. You'll need to remove your Primary attack ability from the A button and have that slot empty.

Remove your weapons and your Secondary skill (either drag it off the toolbar, or go equip your secondary skill somewhere else and it will change to an auto-attack), and hit him with your right-mouse button (secondary).

As a helpful tip to avoid dying, when Diablo gets to really low hp (not sure of the exact %, but its less than 5%) in the final fight, he will stop attacking and simply sits there.

This gives you plenty of time to remove your weapons and skills without risking dying, and to punch him