For this achievement, it asks you to kill the boss or miniboss by smashing (ramming) into it.
It's best done it on Mission 2 'Chemical Brothers', where you only fight the tank boss in the cavern. When you reach the end of the mission, the boss appears. As soon as it appears, lower down its health until it's low. When the boss's health is low but not blinking, descend down all the way to the floor (push and hold Left Stick), go a little far away, ram the boss by moving forward into it. You'll notice that you damaged the boss. Do this a couple of times and you'll get this achievement.
Other notes: If you happened to have 9 out of 10 vehicular enemies killed by smash and you smashed this boss, you can also get the 'Kamikaze' achievement if you didn't get it early.