This achievement can be completed as an Engineer, on any map and gamemode.
You need to deploy the sentry gun damage on the enemy, and then finish him with your shotgun.
For instance, scout run of the sentry, then you just shoot him with your primary weapon with a single shot (because scout players have only 125 HP, and sentry take a lot of it).
It's easy, it's just about to play a lot enginerer.