In order to purchase everything in the game, you'd normally have to play through the single player portion multiple upon multiple times. Not so. You're actually able to load a saved game without reverting back to the number of tokens you had at the time of the save. Any tokens you earn will always stay with you and add to your total, regardless if you load a saved game. You can also change the difficulty level and play a saved game at that new setting, which will earn you more tokens.
The best way to take advantage of this is to play all the way through the single player game on any difficulty setting, making sure you save when you reach the last boss. The last level gives you far and away the most tokens, and once you get the pattern down it's not an overly difficult battle. Every time you load that level, beat it and then save the game, your Battle Token total will increase by a nice amount.
You should earn a minimum of around 30,000 tokens for beating the last boss on Easy level, 45,000 on Medium, and 60,000 on Hard. Once you're able to beat Cerebulon on Easy mode, bump it up to Medium and then Hard difficulty. The fight normally takes around five minutes if you can beat him without having to continue, so it won't take long to earn enough credits to buy everything in the game.