There are coins in EVERY location. During a level, if you find a treasure box, attack it and collect the coins. If you cross a single coin, it is worth 1 coin. If you cross a stack a coins, it is worth 10 coins. If you cross gold brickes, it is worth 50 coins. Sometimes, during a party mission, there is a bonus wave and you need to collect coins as much as you can. If you decide to leave a solo or party mission, it still saves your coin. If you complete an mission, you can get coin bonuses.