Happened to me to, I tried using a thunderclap after and it worked (lightning is secondary weakness). I didn't have the Guan Dao though, and I know you do now, so just use that. Longswords in general.
Oh, I was using Demonbind.... Lol
I want Ceadus armor :c I'm working on Jhen Blademaster, Bhnabara+ Gunner, and Lagiacrus Gunner.
But Ceadus is beast, I'm working hard for it. :o
FAILEDIT: Oh stuff, I just looked it up, Rathalos' weakness IS Dragon. O.o and water barely does anything.