Demons must submit to the will of the living. So all you really have to do is command them to leave.

All normal people are just like demons to me. Godless, lost, insignificant and weak. We're all in darkness but even if you step out of it for a moment it all vanishes instantly.

The light is invincible because darkness isn't even aware of it's existence and vice versa. The difference between them is light exists forever, darkness is only temporary.