If you play engi on MvM, you know the dilemma with Sentry busters. Today I've found a strategy to help you succeed.
You will need: Ubercharge Power-Up Canteen.
How it works:
pick up your sentry and walk far enough away from your dispenser. The Sentrybuster will approach you. When he does, press H to activate your uber. Both you and your sentry will survive.