Portaboy+ I've been loving this as something to play during short study breaks.
For anyone who's struggling, I think I found the optimum starting strategy. After you get the shotgun, a red arrow with a target on it will appear in the background and move across the screen. Shooting this arrow will instantly skip you to round 14 and set your balance to $14, regardless of how much money you had before. Finishing round 14 will begin the spooky mode. To optimize your opening, play until you have $10 and buy ammo before shooting the arrow. If you do everything right, you can start the spooky section on round 15 with $15 and $10 worth of ammo.