How to get a Diablos-Hardhorn?

Diablos Hardhorn is one of the rarest rewards for defeating a G-rank (Black) Diablos.

To even qualify to receive the hardhorn, you must break both it's horns during the hunt. This can be made somewhat easier by getting it to charge into certain 'softer' parts of rock walls, which will cause it to become temporarily stuck, allowing for a number of free hits on it's face/horns.

Once both horns are broken, and the Diablos is killed/captured, you will have a 5% chance to receive a Hardhorn per Diablos (7% per Black Diablos) that had both horns broken.

If you have access to it, the 7* Port quest "[Advanced] Plain Dangerous" is probably your best bet for obtaining Hardhorns if you are able to take down both a G-rank Sand Barioth and a Black Diablos (and deal with possible Volvidon/Savage Deviljho interference). This quest, upon completion, gives you two separate chances of 5% for a Diablos Hardhorn in addition to the 7% chance if you broke the Black Diablos's horns. That's roughly a 16% to get at least one Hardhorn in the reward box (9.75% if you didn't manage to break the horns).

Do keep in mind that even at a 16% chance, sometimes you just get streaks of bad luck. You could run that quest a dozen times and not see a single one, and someone else runs it with you and gets 3 in a single run. Random is random.