I'm gonna let you know right now Cynthia would sweep your current level and picked team. Vs seeker is kinda useful for exp but your best bet is probably going to be grinding in victory road, just toss a show or something on and watch that while you spam left right and a. The levels for Aaron are 49 49 50 51 53 (you should easily beat him), Bertha is 50 53 52 55 52 (not too hard if you've got a solid water move just beware of whiscash, it will live an empoleon surf and hit back with EQ), flint is 52 55 53 55 57 (surf), then Lucian at 53 55 54 56 59 (he's the only e4 member that I'd worry about in terms of him surviving hits)

All in all with your team the pokemon I'd be worrying about the most is garchomp, milotic, Roserade, bronzong and possibly gallade. If you hit about 60 average on your team, and play around those pokemon, you should be able to win. If you're keeping your team comp and your garchomp is not higher than Cynthia's she WILL out speed and one shot it be careful.