Mantyke/Mantine may not be the best water types, but if you want to use it in game you should. Even if he has low attacks and a big weakness you shouldn't have much trouble with in game trainers, so use a Pokemon you like. =P Just switch him out when you battle competitively.
Anyways for the elite four...
On Aaron you should be able to blow through him with your Staraptor if you level it up some more.
Bertha will go down with your Empoleon, but you'll need something with a grass move for Quagsire and Whiscash
Flint's Pokemon will go down with Empoleon too, Drifblim to Luxray and sombody else for Lopunny.
Against Lucian There isn't too much you can do except beat him with your strongest attacks (since you don't have a dark type). Crunch, if you have it on Luxray, is good. For Medicham use Staraptor.
For Cynthia- Milotic goes down to your Luxray, Roserade to Staraptor, Dialga can take out Garchomp. As for her other 3 I don't know what to tell you. Gastrodon, again, you need a grass type. Spiritomb has no weakness. Lucario has a lot of weaknesses (fire, psychic, ground) but I don't know if any of your Pokemon have those moves. I suppose Dialga could cover those though. Lots of people say they get throught the elite four using him =)
Just level up a bit more and maybe get a temporary grass or psychic type to use. Hope That helps.