The worst part about this game is the allowance you get of coins. You can have really strong units, but if you run out of coins mid fight, you can only fork over cash or quit.

You can use gems to replenish your 3 master marks, one of which is 5 free spins for a mark. But thats a waste of gems needed for summons (lets be honest, the better the units at the start, the easier later.

You can also send them on quests to get coins every 4 hours, but you should have like at least a mil before doing any quests.