This trophy/achievement is a huge grind, as you have to fill out the Sphere Grid for every single character. Doing this also includes all of the empty nodes, so you will need to farm the purple spheres from some of the fiends in the Monster Arena. Since you are going to be doing Penance, if you want all trophies/achievements, you will be pretty close to maxing out most stats for your characters. If you’ve already defeated Penance, then Magic Defense Spheres are the easiest to farm to fill in empty nodes.