The first out is Spiritomb, so Dialga would be good, since Spiritomb's dark attacks are not very effective. For Gastrodon, a good choice would be Shaymin. Grass moves deal 4x damage on ground and water types (plus, if your Shaymin is Sky form, ground will have no EFFECT!). Infernape will be splendid on both Lucario and Roserade. Just to make it easier, if it does not already know it, Infernape should learn flamethrower. I am not sure, but I think I remember Milotic using ice beam, so even if this is a water type, it could be dangerous to bring out Shaymin (especially Shaymin Sky Form), so Luxray is your obvious choice. Staravia is well used against Garchomp, its earthquake dealing no damage. To avoid dragon rush, a double team or two might just about save you. Shaymin Sky form might be fine if you don't feel like training Staravia.