Complete all the Professor Genkis arenas.
Note : Health Regen is disabled. You can only heal by shooting Health targets.
Pretty straightforward for the easy and medium ones, just shoot your way out and you should get the required cash easilly.
For the hard one, you'll need to focus more to get the cash required to leave. Headshot the mascots, shoot all the Cash and Ethical targets. You also get $50 for evading traps.