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.