Only use one coin at a time (Don't stack them). It will disappear when you defeat a boss. To be safe wait 15 minutes from after you defeated the boss before activating one more coin. Each time you don't get an exotic the next coin you activate will give you a higher chance from the next boss kill. Using this method you should expect to see maybe two exotics for every 12-15 coins used. The time you have to wait is because if you activate it sooner the game might think you are trying to farm with the coins.