The achievement is pretty simple, you have to deal 2500 damage with Boris Sentry gun. However unlocking it in a public matches can be very difficult as the turret is very weak.
The easy way to obtain this Achievement is to load up a custom portal experience and then farm it against bots.
I've made the custom settings so you just have to load it up with the code: aanu1a
Incase code no longer works you can make your own experience, i used following settings
Map: orbital
15 friendly AI
49 enemy AI
Beginner PVE AI
Video: