During a mission during Act 2, giant walking mechs will show up. To pull a pilot out of his seat, you'll need to throw cars at it to stun it. You'll have to be in the black suit to throw cars. If the car has a civilian in it, you'll get 2 Black points as well.
The first time you stun it, you'll have to run up to it and hit X to start a quick time button pressing sequence. This will rip its gun off.
The second time you stun it, you'll get the X prompt again. This time around, you'll need to hit A as rapidly as possible. At this point, the pilot will get thrown out and the self destruct will be activated and you'll get your achievement.
If you don't get it during the mission, they will randomly appear in the world until the end of Act 2.