I think this game should be a more active one, I know it's TD but at least the monsters could be fancier and more original I think. A bit more effort on the look of these please, I want to see something really strong when it is a boss, not just a cockroach.
Exactly said by insaneric. Equip lightning, even at the very beginning go to the top of the boss, just press and hold fire. It will be dead before it moves.