There are two basic Swampert sets: Cursepert and mixed Swampert.
Cursepert:
Swampert @ Chesto Berry
Impish Nature
EVs: 252 HP, 252 Def, 6 Atk
- Curse
- Earthquake
- Waterfall/Avalanche
- Rest
Basically, wait until his counters (mainly special sweepers) and hazers are out of the way, curse up, and continue to wreck havoc.
The other Swampert is a bit less original, but it works, and also makes up for his huge (yet only) weakness:
Swampert @ Leftovers/Chesto Berry
Relaxed Nature
EVs: 252 HP, others are up to preference
- Earthquake
- Surf
- Ice Beam
- Rest