Recently, with my original Reborn game, I also thought about beating Amaria, while assuming the field will be indeed The Water Surface (and, by extension, the Underwater Field), and I came up with a few ideas concerning Electric Pokemons.
One of my choices, as a team Reserve, is Heliolisk. More precisely, Heliolisk with Parabolic Charge and Dry Skin as his Ability. Why? With Water Immunity, on top of the passive heal with the Field + Ability (on top of Leftovers, if you have), plus heal with Parabolic Charge, with a nice boost due to the Field (Plus your other Electric attack), he could last very long in this battle, while taking out every non-Ground Pokemon (if your Heliolisk has HP Grass, well, it could help).
I also came up with Eelektross. Thanks to his ability, is speed isn't halved, plus his nice coverage (and eventually Giga Drain if the Tutor is here, if not, Grass Knot) could allow him to perform well.
Edit : And while writing, I thought about Lanturn with Water Absorb, who could be a nice idea. But expect Amaria to have it too.
Edit2 : Well, I just realized while checking my Mons, and Heliolisk actually learns Grass Knot. No worries about getting the Grass HP.