Raiden V: Director's Cut (EP)

Sols are hidden collectibles that are buried down on the ground but there is one in all stages, except for Stage 4. They are these silver triangle ones that you must hover over it for to rise up, then shoot it. You can replay Chapter 1 or additionally Chapter 2 and work both towards the Fairy Rescue trophy. Make sure that you end at least the scene you are on before restart, otherwise your progress may not save. Also note that there is no telling how many you have!

See Miklas for some videos.