Something else you could try doing is a rebirth/class change system for your troops. This way it'll add more replay value and you'll get the extreme pleasure of roasting the enemy with this stat boosted class from so many rebirths/class changes.
Need Gold? Get to level 10 and do the Hard missons on the "Help wanted" sign. Get it to say "Boxy Troubles". Throw on your 5x boost and do it like normal. 1~5 seconds before its done, exit and do the mission again. Repeat until the boost is gone. You should get roughly 20~25k, depending on your swiftness.