Ghost hunting action! I tried my best not to resort to the cheat button, I thought I could get by just with upgrading damage and attack speed. The first appearance of the yellow, bullet shooting ghosts were still the point I couldn't get passed. Each run would only give me enough points for one single upgrade which still wasn't enough to defeat them. After getting unlimited points I still tried to get as far as possible with the least number of upgrades bought. The green ones, especially, really spiral out of control if you don't hunt them down right away, so the speed and jump upgrades are necessary for those guys, plus the health upgrade to survive the swarms they can spawn. In the end I had to fully upgrade absolutely everything to complete the game, pretty tough!