In the Bounty Hunter game mode, players drop "bounties" which appear in the shape of giant coins on the map. The enemy coins grant you score, they are red. These are the coins you'll be aiming for in order to get this achievement. You'll simply have to keep an eye out for dying enemies and quickly rush to their bodies, picking up the red coins as they drop them. It's worth noting that revived enemies do not drop another red coin. It's also worth noting you can pick up coins from your kills or allied kills in order to get progress for this achievement.