The first wave is doable, and maybe the second. Past that it's basically impossible. It's deliberately balanced so that you pretty much have to throw money at it. One kill grants one geode buys one bullet, so you literally can't afford to miss. Geodes are persisted, so in theory you can grind it, but it wouldn't be a fun experience.
I actually made the crack and website first. The original idea was "Beachhead 2000 with microtransactions", but it wasn't bad or weird enough for my liking so I started throwing in other things later on. The bosses get weird around wave 30, and if you can get to wave 51 (impossible without cheats) there's a surprise. In retrospect, though, I feel like I should have stuck to the original concept instead of breaking the facade halfway through.