I recommend using at least one tank in your team like Blissey, Goodra, Slowking (Spdef) and/or Skarmory,Forretress,Ferrothorn,Slowbro (Def) ~ there's a lot.
Using primary status conditions helps a lot (Paralyzation slows down the enemy for a whole 75%, Burning decreases the enemy's ATK for a whole 50%)
It's all about using a decent strategy, the Pokemon don't really matter as long as they don't have serious disadvantages (like using Rock types vs Fight types for instance).
Doesn't mean you can't win just because you can't outspeed a Pokemon.