Can someone help me beat ghetsis in pokemon Black

I would suggest that you use a fast Dragon-Type pokémon and maybe an Ice-Type.

Other than that I can only say that you should have high levels.

first i suggest you to make your pokemon in the high 50s and the low 60s. then you should be fine. his hydreigon is hard so do NOT use a dragon type. he will use dragon pulse and kill you in 1 to 2 turns. i recomend an ice type