Reflect an enemy's grenade.

You can see when an enemy throws or hits a grenade towards you and will have a short time to melee with 'RS' to knock it back. The time you get to knock it backs is generous and even still you can unlock a perk that adds an extra second onto the time the enemy grenades take to explode if you happen to be struggling with this. Luckily you don't need to hit or kill another enemy with this you simply just need to hit the grenade with a melee attack.