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. What you'll be aiming for are the blue coins, which are those that your allies drop once they are killed. When you pick these up you deny the bounty, which saves the enemy team from picking it up for a point. Simply follow your allies around and pick up 100 blue coins after they die and the achievement is yours!