(This is my suggestion/way to win) You start with $500. First, buy a strong cannon. Beat level 1 and continue playing level 1 till you have enough for another strong cannon. Upgrade your cannon to 2 slots, and buy a little seeker (I recommend playing level 4 again and again). Don't buy potions and specials, theres a butterfly that gives you 1 random one. Play a level over and over and soon you will have one of each potion/special. Play the levels till level 7, and get 4 laser cannons, 4 double slashers, 2 strong seekers, and the triple slots for your cannon as fast as you can. This shoudld be enough to finish the game.

Inportant tips:
-Snail cannons, ice cannons, and acid guns suck, I DON'T recommend them.
-The fire gun and flamethrower are good for close range, but suck at long range. I DON'T recommend them if you're a long range person like me.
-I DON'T recommend any potions/specials except health potion (low HP use), Distress call (fast money), and energy potion (awsome damage).
-I recommend double slashers because of its high damage although it's low range.