Naturelly, Any GS with white or better sharpness, Raw power of 1100 or better.
I mostly just run after the devil and start charging as it starts to turn, but I don't always try to go for full charge. IMO scoring a hit on it's head is good enough, I'm fussy so I make sure I broke it all and cut the Rathalos's tail before attacking it routine.
I find a few situations good for aiming at a full charge hit on it's head, It's pretty easy to hit it while timing it's turning(or tail spin) while not in fury, otherwise. I find it good to aim for he head just as it lands after an aerial assault.
Oh BTW, Flash bombs don't always help.. getting the Rathalos's attention makes it easier to aim for it's head(and get hit! lol)
Depending on the version you play, various skill will aid you well.
I'll go for sharpness +1 with ESP mostly so that I do not bounce off it's wings and get better sharpness, affinity is also a good bet.
Add: for MHP2G
Black Fatalis Blade 1344 ATK, Drg 270 1 slot (artisan +1)
Swordsaint piercing
Ceanataur Mail Z 2 slot (Sharpness +1, Artisan +1)
Ceanataur Braces U 3 slots (Artisan +4)
Diablo coil Z 2 slots (Expert +3)
Ceanatuar Greaves Z 2 slots (Expert +3)
My setup for 4 skills, Reckless abandon+1, ESP, Sharpness +1, Sharp Sword, very little resistance (F8,W3,T-2,I3,D0) DeF 538 on Full upgrade