Finish a zone killing 60% of the units on foot.
You can do this easily on the first zone. As soon as the mission starts press to hop out of your vehicle, and make a point of not getting back in as it respawns through the duration of the game. Kill as many enemies as possible before they go off of the screen, especially during boss fights. Go through the lair slaying all in your path, and be sure to use all available special weapons on the bosses to make the fights go by faster. This achievement can be unlocked on any difficulty.