Considering the overwhelming popularity of this game there are lots of beginners so my tip for today is specifically for you.
When playing on the easy level, any spell card will work on any villain. So use a variety of cards throughout your game play, by doing so you will power up your spell cards an even amount thus making them more effective when you are playing on the medium and hard levels.
For example, there is a villain that is weak against princess spells. However if your princess spells are not strong enough, it will take you longer to defeat that villain when playing on the medium and hard levels.
There is a reward by powering up your spell cards, as they become stronger you will notice a change in animation whether its an increase in projectiles or a combo of different animations for that particular character.
So for all you newbies out there make sure you’re mixing it up! Variety is the spice of life!