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 done by grinding against wild Poochyena and Carvanha. You ay 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 bit of a stronger Water move.
Putting this into a set, I'd recommend something like this:
Swampert @ Mystic Water
EVs: 252 HP / 252 Atk / 4 SpA
- Brick Break
- Ice Beam
P.S. Please keep in mind that to fully max out EVs, you'd have to lower the unnecessary stats using EV-reducing berries, which can be a little hard to come across. Additionally, grinding against all those Pokemon could take longer than it would take to beat the Elite 4 without EV training.