Guys. Assault Vest Clefable.
with
Calm nature, Unaware ability
EVs: 252 HP, 4 SpA, 252 SpD
- Clear Smog/Circle Throw
- Moonblast
- Thunderbolt/Thunder
- Fire Blast
Consider the following calc:
252 SpA Swoobat Stored Power (260 BP) vs. 252 HP / 252+ SpD Assault Vest Unaware Clefable: 154-183 (39 - 46.4%) -- guaranteed 3HKO
252+ SpA Swoobat Stored Power (260 BP) vs. 252 HP / 252+ SpD Assault Vest Unaware Clefable: 169-201 (42.8 - 51%) -- 3.9% chance to 2HKO
Not even a Really small chance to 2HKO. Now, here is what Clefable can do back:
4 SpA Unaware Clefable Thunderbolt vs. 0 HP / 4 SpD Swoobat: 200-236 (72.7 - 85.8%) -- 75% chance to OHKO after Stealth Rock
Or, if you're okay with being risky:
4 SpA Unaware Clefable Thunder vs. 0 HP / 4 SpD Swoobat: 244-288 (88.7 - 104.7%) -- guaranteed OHKO after Stealth Rock (else 31.3% chance)
Also:
4 SpA Unaware Clefable Clear Smog vs. 0 HP / 4 SpD Swoobat: 56-66 (20.3 - 24%) -- possible 6HKO after Leftovers recovery
The choice between Clear Smog and Circle Throw is up for debate. I have calculated that the combination of Clear Smog and Thunderbolt has a 59.375% chance to KO barring crit chance, but Circle Throw is better for if your opponent decides to switch or if you have Stealth Rock up. Moonblast is STAB, Electric move is for Swoobat, and Fire Blast is for additional coverage.
About Quagsire...well, he isn't so fortunate.
252 SpA Swoobat Stored Power (260 BP) vs. 252 HP / 252+ SpD Assault Vest Unaware Quagsire: 187-222 (47.4 - 56.3%) -- 84% chance to 2HKO
Here's the best he can hit back with:
4 Atk Unaware Quagsire Ice Punch vs. 0 HP / 0 Def Swoobat: 154-182 (56 - 66.1%) -- guaranteed 2HKO after Leftovers recovery
Still, I agree with bobbyvaporeon in limiting S-moves to one per Pokémon. Even then, Swoobat only loses out on Secret Sword (which is quite a loss in terms of dealing with Dark-types), and even without it, it's still pretty threatening. Oh, well. That's just my two cents. The rest is up in the air.
EDIT: I also like Yayz's idea.