Special Missions with 100 boost points. The boost allow the missions to only cost 5 energy and they give good uni XP.
If you have a meta character that can smash Story 13-3 it’s a single boss fight and you can beat it quickly. That’s probably the best and quickest route.
Every mission gives the same except chapter 11-13. They give more but cost more energy so it’s the same. Dimension rifts don’t give any. So story missions are the best because they cost less energy, can be rubbed without boost point without losing much and also give shifter bios.