Training

I really want to do the battle arena a lot. But training Marapets can be hard. I can go to the gym, but that takes an hour, and it costs crystals. To get crystals, you can do Haunted House quests, but those can get pricey and you may not get the right crystal. Buying crystals is also pricey. Does anyone know a quick and easy way to raise health, defense, and attack stats? If so, let me know!