Turn the difficulty to "beginner" level, which sets the number of rounds to win a match to 3. Then play arcade mode until you get the Pit (night) to come up as the first stage. The goal is to unlock the hidden fight with Classic Reptile, so if there are no silhouettes flying in front of the moon, pause the game and select the option to pick another character (even if you will just use the same one). After selecting your character, the fight will again start in the Pit. When you eventually get a match where you see the silhouettes flying in front of the moon, get a double-flawless and perform a stage fatality. You will unlock the fight against Classic Reptile. Battle Classic Reptile until the end of the match. When the announcer says "Finish Him/Her!", pause the game and pick the option to select a new character. When you begin the new fight, it will introduce the original character you fought to get to Classic Reptile as if they are a secret battle that you've unlocked. Each round you win against them nets you 5,000 koins, while each round lost earns you a still sizeable 1,000. When the announcer says "Finish Him/Her!", pause the game and choose to select a new character. Rinse and repeat. This glitch allows you to continue to fight the same character, each time earning 1,000 or 5,000 koins per round as if you are fighting Classic Reptile. Assuming you set the rounds to win a match at 3, you could earn 12,000 koins each match (2 won and 2 lost rounds)! This will make it incredibly easy to collect the 299,880 koins needed to unlock everything in the Krypt! 25 matches with 2 won and lost rounds each will do the trick.