Kill 10 vehicles by smashing into them (accumulative).
For this achievement, you're required to descend down (push and hold Left Stick) and ram again the vehicular enemies. It can be done on any mission; for me, I personally like to do it on Mission 2 'Chemical Brothers', as I can see a couple of enemy vehicles both outside and inside the cavern. This can be a little tricky as if you hit the enemy with your ram or not. To make it noticed that you hit them, descend all the way (showing where your chopper lays on the ground) and ram them a couple of times per enemy. You can lower their health by hitting them with machine gun a little bit and ram them for a little quick smash kill(s). Do this ten times and the achievement pops.
If you happened to have 9 out of 10 vehicular enemies killed by smash and you smashed the tank boss (found at the end of Mission 2), you can also get the 'Bossfight Like A Boss' achievement as well.
Note that only choppers' ram can allow the kills to be registered. Using the tank (on mission 6) and ram on any vehicular enemies doesn't count.