If you are having trouble with the sudden death levels, keep in mind that the ranged units usually cannot hit you as long as you keep moving, so just run around in circles and use your special abilities.
There does not seem to be any way to get from a level to the menu. Also, if you have cheats on, you can spawn zombies and kill the love-zombies in the screen leading to the main screen thus stopping you from ever reaching the main menu....
 "Mighty Knight"){kind=icon}
 "Kids Vs. Santa"){kind=icon}
 "Flappy Bird Flash"){kind=icon}
 "SuperBikes Track Stars"){kind=icon}
 "Madville"){kind=icon}
 "i saw her too, with lasers"){kind=icon}
 "Twinkling Sky"){kind=icon}