I beat all the levels and they get pretty damn tough toward the end especially when you have to start juggling major flying spawns and ground guys. The later levels do not give you such a well defined path either so you have to make huge diversions with towers. I know I used gun towers until the end but also cannons are incredible when fighting the bigger guys and they hit flying dudes as well. The flame towers are great in choke points with a slow tower.

Yeah, just be sure to get very familiar with all the towers because you will need them all during the last stages.