How to Quickly unlock Arena mode without needing to beat the game.
Quick Unlock Method
You can unlock both mode within 10min of starting the game.
You'll need to get the sandbox key first: You know that secret key you got for Sandbox mode? After you complete the other guide to get the key. There is a crate in your Main menu that you break open and get the Sandbox Key. Well it's actually a universal key. On the main menu where you would plug the key. there are two plugholes.
One that says A Fantasy World, the other says Sandbox. The Sandbox key is universal and you can plug it in to either hole and then play that mode.
A Fantasy World is the Arena Mode.
After you plug them in one time, the option stays unlocked so you dont need to keep switching.