Roll to avoid lethal fall damage.

This achievement means to avoid damage, not death. That means if you are too high up, you will not fall into a roll and will die. The first spot to obtain this is listed below. As you are falling, you must hold LT to roll once you hit the ground. It is very easy, you just need to find a spot to do it.

This can be obtained in the first level. There will be a point when you find yourself outside and have to climb up ladders. Once at the top, you will have to make your way down the other side. There is a ladder that leads down. Instead of going down the ladder, press A on the railing to hang on it. Press B to let yourself drop. Hold LT the to roll and gain the achievement.

You will later be taught how to do this, and likely get the achievement there if you had not gotten it yet.