How do I get past Ghestius (or N's partner) in pokemon black version?

Um, okay. Pikaswag, you need to first remove the Excadrill temporarily, then train... and train... and train the rest. There should not be one Pokemon that is much higher-level than most of the others. And why on EARTH are you calling Ghetsis "N's partner"? In case you were not paying attention to what he said to you, he was just using N and the rest of us as his freaking TOOLS. So, train those Pokemon and someday you'll beat that jerk.