i found the biggest boss achievement. i dont know if it was a fluke, but i just continously kept heading into the northern bubble. and you will know when you encounter the big boss. he is huge as hell, he spawns little fighters, and is a bitch to kill.
 "Four Second Fury"){kind=icon}
 "Ninja Gardener"){kind=icon}
 "Bubble Tanks"){kind=icon}
 "Desktop Tower Defense 1.5"){kind=icon}