Rathalos isn't all that difficult after you study his attacks and learn his patterns (like all monsters in the game. Knowledge is power!), but boy, is he annoying, because the fight just takes so long. He spends far too much time flying, making it pretty much unable to attack him. Be prepared for the long haul, especially if it's single-player you're facing him in.
MANTI5, your name sounds familiar. Have I seen you in Animal Crossing?