If you need to get money really quickly, either do level 19 (using at least a spread, get a hundred gold quickly, and then use smart bomb when screen is full, you may need around 3-4 shields to do this properly, but you get a comfy 700-800 dollars) or do level 24 (you should only do this if you need quick money for boosting shields and attractor, because its hard to get kills without at least a ion flux, but if you have one, you simply need to stay around the middle point and hold the fire button and dodge the occasional bullet).
Particle Wars Extreme < Bullet Heaven
Kudos to ZEGMAN, this is a awesome game, but after beating the last boss on bullet heaven, there's just no damned comparison...
*hits the start button* *runs into a wall* *stares at the crudely written "YOU JUST FAILED MY FIRST GAME"* *laughs a little* *says to the computer in the small hope that the developer gets my message "You Sir. Are A Douche-bag. A Douche-bag. A. Total. Douche-bag."* *Plays some other game*
 "Racing Comrade!"){kind=icon}
 "Infectonator : World Dominator"){kind=icon}
 "Raze"){kind=icon}
 "Space Arcade (the game!)"){kind=icon}
 "nyanPARTY"){kind=icon}
 "Clockwords: Prelude"){kind=icon}
 "Magi: The Fallen World"){kind=icon}
 "GemCraft Labyrinth"){kind=icon}
 "Elephant Quest"){kind=icon}
 "Damn Birds 2"){kind=icon}
 "Darkest Days"){kind=icon}
 "Particle Wars Extreme"){kind=icon}
 "Insectonator"){kind=icon}
 "Blue Knight"){kind=icon}
 "Windows E.V.I.L."){kind=icon}
 "My First Game"){kind=icon}
 "Hardest Game in the World 10"){kind=icon}
 "Reclamation"){kind=icon}