Instructions
Equip armor with fire resist. This will decrease the amount of damage that your character takes when getting close to Rathian during attacks. Open your menu screen by pressing select. Choose an armor with fire resist. Now you can approach the Rathian.
Dodge and block as many attacks as possible by moving your character to the left or right seconds before the attack lands. Rathian will use tail whip often so you should move when the tail moves. Dodging allows you to determine the Rathian's pattern of attack. Use potions to sustain your health.
Attack the tail. After the tail whip attack, move in closer to attack the tail. By destroying the tail, your character limits the tail whip counterattack. This will allow for more close range attacks on the face and wings.
Use traps to deal large amounts of damage. After the tail has been severed, use shock trap and pit trap to hinder its attacks. Use this small window of time to attack the Rathian. The traps allow more close range attacks. After a few traps the Rathian will be destroyed.