Competitively speaking
Focus Sash
Swoobat has terrible defenses so it needs to set up at least one Calm Mind before it can really start sweeping, something Focus Sash can help out with. Calm Mind + Air Slash / Heat Wave / Psychic / Giga Drain is the standard set and works well with Simple.
However, seeing your in-game-team tag,
the choice is Leftovers, for two reasons,
- Most AI attacks can't OHKO Swoobat unless strong opponent and super effective
- I'm 99% sure Focus Sash is a one time use item that will disappear, meaning you will have wasted it.
If you mean wi-fi battling, the Focus Sash + Calm Mind is always the better play.
