Spec Ops: The Line

I did not get this achievement on my first playthrough, simply because the game does not allow double tapping ''A'' button to count towards the achievement.

-You have to HOLD ''A'' while running and run towards cover, your player will automatically slide into the cover, tapping ''A'' to sprint and pressing it again while close to cover does NOT count.
-''Under fire'' you need some enemies around firing at you for the slide to count. Play any level on the easiest difficulty ''Walk on the Beach'' for this to be really easy.
-Since its on the easiest difficulty, slide once, walk back a few paces and sprint again (you need some distance for your player to slide), repeat 10 times and you've got your achievement.

All this requires you to do is while in a firefight, sprint into cover (hold A and sprint towards cover until he slides into cover). The good part about doing this is you can just keep sliding into cover, stepping back and sliding back into the same piece of cover 10 times until you get the achievement. As long as one enemy is still alive so that you're still technically in a firefight, you can keep doing this.

The best way to get this is in Chapter 14 when the current objective is: Up the Street - Move!

There will be a cutscene where a mortar is shot and when you gain control you will want to/have to shoot down the mortar team with an RPG. After this you will be between two heavy machine guns shooting at you. Behind cover kill the enemies that are behind the sandbags to your left from the mortar team you shot with the RPG. When this is done you can just go between sandbag covers (holding A while doing so and then hit B when you are in front of the other) and get this all in one go. Be sure to regenerate your health when you're behind cover.

This is best done on the easiest difficulty, I did not die once while running between covers but you do take a lot of damage.