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Monster Hunter Tri | Wii
The Rathalos is the easiest to fight once you figure out it's moves. To fight a Rathalos, I'd just use the armor with the highest defense and no negative fire resistance (Rathian or Barroth armor?). For your weapon... Only sword and shield? Why is that? Anyways, use whichever one you think would help the most. It's really more of a choice between poisoning the monster, paralyzing the monster, or more damage with the water element.
The first time's the hardest; Good luck!