Nice team dude here I can give you some tips for battling the Elite 4:
1.Don't forget to pack up revives,Hyper potion,Full heal
2.Teach your swampert some devastating moves such as Earthquake,Blizzard,Brick break, Toxic
3.Gyarados can learn bite which you can use to crash out phoebe's ghost type pokemons
Note:Beware of phoebe's sableye!
Hope this helps :D