Make 50 kills with the Bukhanka.

You will get the Bukhanka during the Caspian level, its location detailed above in the "Driver" description.

To get a kill with this vehicle you will need to run enemies over. This achievement will almost certainly need to be grinded out, since the chances of find 50 enemies in the open where you can run them over are very slim. The best way to grind this is to find a group of enemies, preferably the chameleon enemies that blend into the roads/landscape and then make a quick save. Then run over as many of the enemies as you can, then reload your quick save and repeat as necessary. The achievement tracker will keep going up, despite loading an earlier save.