REMEMBER: Taunt can go on the sets above to be different from other Swfit Swimmers.
A true Defensive set with:
Set Name: Defensive Pivot
Qwilfish: @ Leftovers / Black Sludge [Item Clause / Mono Poison Team]
EVs: 252 HP / 252 Def / 4 SpD
Nature: Bold (+Def, - Atk)
Ability: Intimidate
Spikes / Toxic Spikes
Boiling Water / Taunt
Pain Split
Substitute / Stockpile
Switch in Qwilfish on a physical attacker/wall, Intimidate, cause a switch and Substitute. Use the Spikes of your choice as your Substitute takes a hit, then Substitute back up.
Even with Max HP, it only has 334 HP which isn't so high, especially after a few hits, making Pain Split usable. Boiling Water is usable for lowering the foe's attack further through some Burns, and Spikes allows it to take advantage of it's defensive role and cause some entry hazards. Toxic Spikes can be used, but at that point it would be better to just use Tentacruel. Taunt can also be used, but would be better on a faster set.
The choice for the last move is really dependent on your team, Stockpile works like a Weezing set would, as it increases your defenses, and is especially useful if the foe you Intimidated switches out. Substitute is used as the foe switches out, so you can scout, and safely lay down Spikes.