The game had been a lot of fun, up until around zone 6. Specifically the 6-5 boss. The game almost suddenly increases the difficulty to impossible, unless you spend a significant amount of money. I have gotten every medal in the game up until zone 6, and have even spent a small amount of money. Still, the upgrades achieved by literally 100%ing the game up to this point and paying for some crates, are not even close enough to beating this level. It seems impossible to make any progress forward.