I tried harvestsun's tactic, and he was spot on with the upgrades, but to get the most money, starting level 3 to 9, shoot a very small amount of cannonballs(and hit with them) to get a 100% accuracy rating after the level. You see, starting from level 3, onwards, a 100% accuracy rating gives you more money than the 2500 gold they give when you don't shoot a thing.
Tip: When enemies are scaling a tall stack you've made and are near the end of it, freeze time, sell that whole stack, unfreeze time for a very short while, freeze time, and the demons will be at the bottom of the stack. This can be done around 2-3 times.
I'll admit that this seemed a lot funnier 6 years ago. The whole Kongregate badge system is basically a time capsule for my youthful immaturity.