Pop 17 Balloons.

You will find balloons floating around in the sky as you explore the world. Their appearance is random so it is best to shoot these whenever you see them to make progress on this achievement as otherwise, grinding this out at the end will be very monotonous. The balloons drop a crate with loot in them, however you don't need to collect the loot or reach the crate, simply shooting and popping the balloon is suffice.