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.