Sometimes it's possible for scouts to just run past your sentry with very little damage. If you force him to zig-zag past your dispenser, then he won't make it that easily. A bomb . Maybe a soldier or demoman can position himself at a good spot where they can hide from the sentry and shoot at it. Spy spotter .
Engineers just hate spies.
When in a fight try to hold the primed EMP for a while before throwing it, making it harder for the enemy to dodge it. This goes for all grenades. Don't just throw the grenade and dodge away. Blowing up sentries . Yes it will take out a fully loaded sentry.