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First things first, in order to be able to reflect grenades you'll need to unlock the Grenade Tennis perk in the Kill and Destroy project skill tree. If you haven't unlocked the skill tree yet, you can do so by completing the story mission "Blackout" which can be found in the town Gunbarrel.
Once that's done, you just need to bat a grenade out of the air. When an enemy throws one at you make sure to melee it while it's still in the air. If you have trouble doing that, invest in the next skill in the perk tree which will allow you to melee grenades even after they've hit the ground.
Last note; enemies throw grenades in this game like mad, so you'll have no shortage of opportunities to get this achievement.
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