The second last level is possible by using Assassin with a Barrett and making sure the enemies keep being in the air, if you want to beat it without spending time leveling up. Also rip flash.
Maybe you shouldn't have made the drivers robots that rely on 3 short range sensors to drive recklessly, thus making high top speed guaranteed death. What would of REALLY fixed it is to make the sensor in front of the car get longer the higher it's top speed is so it doesn't ram straight into a wall at Mach 10. Thank you for listening to my ted talk about how sensors can work if made correctly. Sincerely, someone who makes comments which are way too long.
You've all heard of sticky homing sheep cannon with 80% armor pierce. now get ready for: AUTOMATIC DOOTY LAUNCHER, one of the only things general is good for.
How to break the game:
Step one: Get charge tower (Or similar) and two fire rate towers and a range tower all at max upgrades at the start of the level. Can't? Then spam click the ever-living ???? out of the send wave button and then max upgrade the towers. Level beaten in less then 5 minutes or more depending on the amount of rounds.