Counter that Pokémon

[Nah]

Swampert is far better as a physical tank.

As for the counter:

-Cradily w/Leftovers
Nature: Relaxed
Ability: Storm Drain
EVs: 252 HP, 252 Defense, 4 Sp.Defense
Moves: Giga Drain, Rock Slide, Recover, Toxic

Is immune to Surf, can sponge Earth Powers, and OHKOs with Giga Drain.

 

[Megan]

Lucario @ Life Orb
EVs: 252 Atk / 252 Spe / 4 Def
Adamant Nature
Ability: Justified
- Close Combat
- Bullet Punch
- Extrem Speed
- Swords Dance


0 Atk Cradily Rock Slide vs. 0 HP / 4 Def Lucario: 22-27 (7.8 - 9.6%) -- possibly the worst move ever
0 SpA Cradily Giga Drain vs. 0 HP / 0 SpD Lucario: 45-54 (16 - 19.2%) -- possible 6HKO

252+ Atk Life Orb Lucario Close Combat vs. 252 HP / 252+ Def Cradily: 367-432 (97.6 - 114.8%) -- 81.3% chance to OHKO

 

[Zeffy]

Aegislash @ Leftovers
Ability: Stance Change
EVs: 244 HP / 12 Def / 252 SAtk
Quiet Nature
- Shadow Ball
- King's Shield
- Sacred Sword
- Iron Head