Grass, water, steel, ground, and fighting types are all super effective against rock types, leaving them quite vulnerable.
They are good against bug, fire, ice, and flying. If you need a pokemon to greatly damage these types, a rock type is a good choice. They usually have high defences. Some good rock types could be Rhyperior, or Archeops (Archeops has low defence, but high attack and speed). I do not know whether you need a rock type on your team, since I do not know what pokemon are in it. Hope this will help you!