Diablo is for some reason the toughest of the three brothers to kill.

I've used every character to fight him and only one method works: Blitz.

Before going after Big Red you want to create a PORTAL in the area near the SEALS (Diablo can't or won't go up there or attack there) and check your POTIONS, run out find DIABLO and hit fast and hard and KEEP MOVING! NEVER STOP MOVING!

The second he starts retaliating start guzzling POTIONS. When you clear out the first two columns head back to your PORTAL and stock back up at the PANDEMONEUM FORTRESS.

Go back through the PORTAL and create another PORTAL.

Repeat. Repeat. Repeat. Repeat.

It'll take some time but you will eventually get him.

if you want to kill diablo easily you need a weapon that have a lifesteal at least 15% per hit.... and put Diablo on a corner, and you need a lightning and fire resistance MAX if you do this to Diablo you can kill him easily!!^______^ hope it help!!