To take advantage of Swampert's Attack and powerful Earthquake (you do have Earthquake on it, right?), you'd be best investing 252 EVs into Attack. This is best achieved by grinding against wild Poochyena and Carvanha. You may also want to max out HP, to ensure your Swampert can survive an attack. HP EVs can be grinded by fighting wild Whismur in the Rusturf Tunnel. The remaining 4 EVs don't matter at all; you can give them to Special Attack if you want a stronger Water move.
Putting this into a set, I'd recommend something like this:
Swampert @ Mystic Water
Ability: Torrent
EVs: 252 HP / 252 Atk / 4 SpA
Relaxed Nature
- Surf
- Earthquake
- Brick Break
- Ice Beam
P.S. Please keep in mind that to fully max out EVs, you'll have to lower unnecessary stats using uncommon EV-reducing berries. Additionally, grinding against all those Pokemon could take longer than it would take to beat the Elite 4 without EV training.