Know which pokémon are better at attacking: Remember that type trumps CP, but, in general, use pokémon like:
- Snorlax
- Dragonite
- Lapras
- Vaporeon (even after the patch)
- Arcanine
- Exeggutor
Leave your best defender behind: The best defenders to place in an empty gym usually have the most health, so they can last longer against attackers. The best defenders, in that order, are:
- Lapras
- Poliwrath
- Dragonite
- Snorlax
- Exeggutor
- Slowbro
Taken from:
http://lifehacker.com/six-tips-for-controlling-all-the-pokemon-go-gyms-in-you-1785369171