Instructions
Use a long-reaching sword to cut off a Diablo's tail. Diablos have a figure-eight tail attack that can easily decapitate you if you're caught in its path. With the tail out of the way, you can focus more of your efforts toward killing the Diablo versus defending yourself from its attacks.
Brace yourself against a wall and hold up your shield if a Diablo charges you. If you're lucky, the Diablo's horn will slide off your shield and become embedded in the wall, after which you can easily hack it to death.
Swing a hammer at a Diablo's feet to quickly knock it over. Once it's fallen, hack at its legs to weaken its ability to get back up. Once it has been paralyzed, slam the hammer at its head a few times to kill it.
Acquire a very long-reaching and sharpened lance. Though lances are difficult to aim, they can do a great deal of damage in a short time, especially if you aim for the Diablo's chest area.
Throw sonic bombs at a Diablo the moment it begins to dig into the ground. This will temporarily stun it, giving you ample opportunity to slash away at it a few times. You won't be able to kill it, but you should be able to cuts it health points in half.