When i get home, i'll post the guide i used. But to describe, chapter 14 (the witch hunts) where you use the rider, you need the gaze of despair (enrages enemies) and the bracelet of time (witch time at your own free will), you PPP(hold) combo the enemies in witch time, you can rack up about 400-500k halos in the first 2 verses, skip muspelheim, verse 4 where you are on the bridge, you jump P, and then keep jump canceling P, it keeps up your combo, and prevents enemies from falling off the bridge, you need to trigger all the enemies and gather them together.
First boss, you PPP(hold) combo til dead, 2nd boss while flying, you just spam Y to shoot bullets til dead, last boss same thing, spam Y bullets til dead.
Things to note, you MUST use Jeanne (2x witch time combo modifier)
Practice to keep your combo up and not drop it.
Always activate bracelet of time (L button) before your first PPP(hold) combo.
If you practice this well, you should be netting about 2-3mil halos every 6-7 min.
I pretty much posted the whole guide off the top of my head, but the video i'll post later has a lot more details about where you need to place yourself to upkeep your combos.