This was pretty fun, though I feel like the mobs didn't give you enough money. Specially towards the end when they were so damn spungy. I had to turn the difficulty down to easy on the last few levels... though I'm not 100% sure that actually did anything?
Turning difficulty down lowers the health of enemies, so it should have a good effect at making waves easier to clear.
One thing I notice about your tower setup is that you have a ton of fire and water towers. These towers are good at igniting enemies and slowing them down, but are not the most powerful in terms of raw damage output. I recommend experimenting with different towers to see if you can get an easier time. The stone and tesla towers can be particularly useful for raw damage output, depending on the situation.