I originally boxed Reshiram, and won the N fight easily without it. However, when my whole team fell to Ghestis' Hydregion, I knew I needed to pull out the big guns. So, I went back to the E4 Pokecenter, got my dragon, went all the way back up N's Castle, and refought Ghestis. This time I flew by him, it really was not trouble. Now Reshiram is a proud and strong member of my team :)