I haven't done Diablos in a while, but I remember when it goes underground turn around and wait for it to go back up, dodge when he does, then quickly go behind and do one or two X strikes to the tail.
EDIT: Rathian is SUPER easy with long sword, strike after somersaults, strike when fireballs are thrown, strike (carefully) when it's twirling around, strike after it somes running at you and misses.
I think you may need to read my posts again. I don't have any problems
at all using the Longsword or Lance. I have MEGA problems using the
Great Sword. I can't get charges off on anything. I have no problems killing with it, but I simply cannot get charges, and I feel like I'm spending more time healing or chasing something than actually hitting it.
This is how the typical scenario has been going:
1. Run up at the start and try to position myself for a charge, or even just a normal attack on the tail: monster instantly charges one of the Shaka partners, or instantly does a 180. SWISH
2. It's standing there, focused on a Shaka partners, so I run up, line up a shot, then it charges all the way across the screen. SWISH
3. It keeps charging back and forth, back and forth until finally it drains itself of stamina, and it's taking a break. I run all the way up to it, line up a chage and then it's tail just so happens to idle animation just out of range as soon as I get a level 3 charge and let go. SWISH
4. Finally get a decent attack in, but then I'm bounced into the wall directly after, set on fire, then dizzied. I finally recover with 1/4 of my life, then run away while on fire trying to heal. I finally heal and then it's time to do this slow ass process all over again.
Or, I can simply run under it and mash X, A, X+A, A, X+A, A, X+A until it goes out of range again. So much for finesse.
By comparison, the Longsword is fucking elegant and even though each attack might hit for less damage initially, I spend a hell of a lot more time actually hitting enemies than I do hitting air. The Lance on the other hand, is like baby-mode and you're practically invincible. It's pretty damn annoying too, considering I just upgraded to a Sieglinde.