Forum:Pewter Gym

Even those who picked Charmander can do this with ease. If your starter is:

Bulbasaur - Have it know Vine Whip. It'll tear almost everything apart. Try using a Speed debuffer (String Shot).

Squirtle - Same above, just bother having a debuffer and Water Gun.

Charmander - Here it's a bit complicated. Have Ember on Charmander. Get a Beedrill, a Butterfree and a Pidgey, using String Shot, Confusion and Gust, respectively. If you feel you need it, use TM to teach Butterfree the move Whirlwind. Make sure they're all level 10 at least (exactly 10 if you want the achievement). Put Charmander and Beedrill on the front. Straight from the entrance, put Butterfree. Pidgey should stay on the back to kill whatever survives, preferably on the side Beedrill is since Beedrill is not attacking. When Onix comes, take off all, and start by debuffing them so they'll get a slower pace. Put Butterfree and Charmander ASAP and wait.