I really need to pick this back up. 45 hours in and I got frustrated that I couldn't chop off Diablos' Tail for the Tailcase that he has. I need 4 of those suckers too to upgrade the weapon I want.
Cycled between the switch axe and great swords but I can never get any hits on his tail. Any hints?
-shock traps
-diablos can get stuck in the two pillars and wall in area 8 (wall is harder to use, as diablos has to be perpendicular to it) and the rock in area 4 that's near the entrance to area 6; stand next to this objects and Diablos will get stuck when it charges you. The pillars are quite fragile (Diablos in rage mode destroys them in 1 hit) so it's probably best to use them ASAP.
-attack its feet to knock it over
-pitfall traps; use a flash bomb just after it frees itself and hovers in the air for a few seconds. It will fall down on the ground just like Rathian and Rathalos. The time window is narrow so you will have to sheath your weapon early and sacrifice a few hits.
-sonic bombs when it's underground and not enraged; just like with pitfall traps
-after a charge it will sometimes stay in place and whip the ground with its tail; if you get the timing down you can score a hit
Don't forget to use your upswing (X+A) if the tail is out of the reach of your other attacks.