You definitely need some sort of counter to electric types. A water ground type is a good counter. Since everyone is assuming that it is gen 7 monotype, first we need to see which ones are available (without trading).
Swampert: Island scan
Whiscash: Seaward Cave, Paniola Town, Poni Gauntlet, Poni Meadow, Vast Poni Canyon
Gastrodon: Poni Wilds, Ancient Poni Path, Poni Breaker Coast, Exeggutor Island
Out of the three, swampert has the highest bst. Looking at your team, I think swampert is the best choice. (You can find marshtomp I think on saturdays. https://www.youtube.com/watch?v=LtFMVVgTGBU
Overall, because swampert is a physical attacker, and has a respectable base stat total, try him out.
Hope this helped!
For the set, try this:
Swampert @ swampertite
Evs: 252 attack/252 HP/4 Special defense
Hope I helped!