Only spend cards that will give a bonus to your fighter (same card). By doing this, you will receive six times the experience and can keep all the other cards for future skills.

Learn all the upcoming skills that your fighter will unlock by leveling up. Make sure that you never spend those cards. When first starting, do not spend all your cards; instead progress through the fights and earn coins. If you do not max out all events, return later and earn more training cards.