Hi,
If you have Charmender try to up him to lvl 16 for he learn Dragon Rage.
If you have Squirtle, use water move.
If you have Bulbasaur, use grass move.
If even with any case you do not succeed, it is that you lack of level, therefore you have to take your time and to mount your team. (Approx. 14/16 for each Pokémon).
Good luck !