Clear the game once as Zizou to unlock the ability to play as Princess Riva.
Quick bonus gauge fill
After beating Klinsy (the librarian) and obtaining dia missile, go to any stage and switch to it making sure no enemies are in range. If you use the spell while the cross hairs are not visable the bar will fill slightly. Then all you have to do is keep using the spell until you have all the upgrades you want.