Army of Ruin

Your earned coins are in the lower right corner, and you earn them from chests and from random coin drops from enemies and containers. Just pay attention to your coin counter on the bottom because there's a decent chance you'll need to avoid killing the boss in the final wave until you've earned more coins. What you should actively try to do while progressing through this stage is move around the map as much as possible to collect coins from containers. If you just hang out in the same area doing circles or whatever, you'll get virtually no coins from containers, and likely won't reach 750 by the time you beat the boss. Also, toward the end of the waves, usually an extra chest or two will spawn on the map. Check the onscreen indicators and the map itself if you have it unlocked (see Map (10G)), and make sure to head to any extra chests for some extra money towards this.