i am sttuck also but i am getting lots of weapons while i get ready to do it like
Deadly poison
poison battleaxe
frost edge+
Deathprize
these are all small swords but i'm sure 1 will do iit for me, u need to take max potions, mega potions, potions, ancient potions, a farcaster, u should kill the rathian first, and just concentrate all attack on it