So what's actually wrong with using Jolteon? I use it all the time for Gyarados, it can kill most sub-2900 ones before Hydro Pump comes (if you can get a CP over 2200 - probably not if you joined after Raikou, but then, how could you already have Ampharos?)
Only Magneton and Jolteon have a significantly higher attack score than Ampharos which you find "way too weak". But Magneton is already slower and less resistant and only has 1-bar charge moves.
You can try Kyogre with Thunder, but the water moves are not very effective. Absol with Thunder probably dies before you can use it more than once. Arcanine with Wild Charge is weak to water. Gardevoir with Charge Beam does not have much DPS apparently.
But besides electric types, you can now easily grow Golems with double rock moves which are also super effective into Gyarados. Exeggutors with grass movesets also are very good and resist water, but not dark moves.