There is a very easy way to farm cash and research everything to maxlvl: When the boss in lvl 1 shows up, sell your longrange tower(s) in that lane, replacing it with flamethrowers. He will continue to spawn units while being out of range of your towers.
The "green" levels are complete and utter garbage. Not only are you fornicated when you didn't upgrade some of the useless units you get there (like turtles or monkeys), since it's random you might end up with 10 useless spells (poison, lol) or units in a row causing you to lose.
Farm the bosses for items, walz through the rest. Fist, heal and lightning, rabbits & 'roos took care of everything so far.
When you push to the enemy base lightning always hits all the enemies.