Ugh, grinding for Rathian armor is slooooow.
You need a Rathian plate in order to build the leg piece; and the Rathian plate only has a 2% drop rate. The item to build a chest piece, Rath marrow, has a higher drop rate at 16%, but the only two ways you can get Rath marrow are by cutting her tail off or capturing her; both are incredibly difficult.
Well, at least I have Lego lad to help me.