This achievement is very easy & can't be missed on the very first level of the story.
On Day 1 (the first level) of the game; you'll jump down onto the streets below & kill a few groups of enemies. After that, you will see a truck that you can slide under. Press LT & forward on the analog stick to slide under it. Achievement unlocked.
Note: You can slide right after you land onto the streets, but you can't unlock the achievement until you slide under the truck in order to continue the game. Click the RS (right analog stick) in to see the path you're meant to be following in the game at all times.