For those of you confused about where exactly you meet these trucks, you meet up with them on the map with the numbers 0-19, in other words the world select map. You will see that a couple of the numbers have purple trucks on them. To activate the Meet the Enemy Sequence right at the start of the game, simply move your helicopter up and down the three different areas available to you at the start of a new game. Eventually, you will land on the same number as a purple truck. From there, you simply kill all the enemies, including the trucks they spawn from, and any motorcycles you encounter. Once your sure no other enemies are around, kill the big truck at the end of the road to get your achievement.