Sigilyph can sweep, but it needs to set up enough Cosmic Powers to do so with Stored Power. Sigilyph is rarely be Life Orbed, even with Magic Guard, due to how frail it is and being outclassed by Alakazam. If you really want, you can run a similar set to Clefable:
Sigilyph @ Life Orb
Ability: Magic Guard
EVs: 252 HP, 252 Sp. Atk, 4 Def
- Cosmic Power
- Air Slash
- Stored Power
The best way to use Sigilyph as a sweeper is to spam stat boosts. It could probably run a Tinted Lens set, but to be honest Magic Guard is far superior as an ability.