Begin but dont finish the Side Story "Eulogy for a Saint", 3-7 is a great farming spot for quick friendship and in general finishing stages quickly for whatever reason you need to, once finished it closes forever so dont collect the rewards (or hope that Smilegate brings out a similar stage in the future). Doesn't give you AP though so keep that in mind, the upside here is that its fast and a cheap energy entry in comparison to most other options.