Got my Tour of Duty Badge:
Played spy:
Had fully upgraded sapper and +100% backstab damage on big enemies(It takes out a fifth of their health with one backstab.).
Sapped the medics as soon as they spawned healing the large enemies as so they can't uber. They only begin to uber when attacked. Fully upgraded sapper will sap all of them.
Backstab them all but make sure to look out for what they were healing, it'll whip right around and blast your ass. Which is why you should wait until it turns a corner before backstabbing sapped medics.
Later in the round had 50% blast and bullet resistance, plus 10% speed increase. I would run in and sap a large enemy like a multi rocket soldier, backstab them twice before they noticed and then run around them jabbing at them with the knife while they tried to keep up while taking sustained sentry fire. (We had 3 engineers.)
Our medic believed it was doomsday brought upon by "Roboprincess Celestia", we all believe him to be mad.