In this video guide I will show you how to reflect and enemy's grenade. You will need to complete the mission Blackout in order to unlock Marshall's skill tree first to do this achievement. Once in Marshall's skill tree you will need to buy the grenade tennis to be able to to this. I would recommend buying a few other skills to unlock the next level in the skill tree. Once you do that you can buy the smash grenade tennis to be able to hit the grenade on the ground much easier.
Once you have these skills just go into an enemy base with some goons and wait for one to throw a grenade at you. Once you see the grenade icon find it and melee it by pressing the RS. Once the grenade flies from the melee the achievement will pop.