Oh, and this will contain spoilers if you aren't at least half way through the game.
First off, you need to have gotten to Chapter VII.
Second, they aren't a necessity, but things will go much better if you have the accessories Gaze of Despair and Selene's Light.
Gaze makes enemies more aggressive but also worth more halos, Selena's Light makes the game automatically trigger Witch Time when you take damage (assuming you have some magic built up).
If you don't have either of those then I recommend saving up 100,000 halos and buying Selene's Light. The Gaze can wait.
Third, make sure you don't have your main game saved at a mid-level checkpoint, because loading up a past chapter will erase the checkpoint save. If you do have one, you'll probably want to go ahead and finish that level before saving, then going back and loading up Chapter 7.
Equip Selene's Light (and Gaze if you have it) and Durga's Claws on your arms.
Make sure the Claws are in lightning mode, not fire (to switch, rotate the left analog and hit Y).
Chapter VII is a boss fight against Temperantia, who has 3 phases - arms, body then head.
Phase 1 - You'll be on a floating platform and the boss will latch on to it with one arm and shoot stuff at you with the other. Get close to the attached arm and wail away with the YYYYY and YYYYB combos. Dodge if you have to, especially when the boss smacks your platform (you'll know he's about to do this because his unattached arm will make a fist and glow with lightning).
After destroying one arm, repeat for the other. When your platform is destroyed, don't panic--you won't take damage from falling, you'll just land on another platform. If he starts to pound with both arms then get ready to jump to another nearby platform as this is easier than dodging & not doing damage.
Phase 2 - After both arms are destroyed, you'll have to run to different orbs on Temperantia's body to destroy their energy tether to his head. After smacking an orb a few times, watch for glowing blue arrows to appear above it and get ready to dodge them. Abuse the same combos, YYYYY and YYYYB. As soon as you destroy an orb, beast run (double-tap Right Trigger) to the next one and repeat until all 3 are destroyed.
Phase 3 - Here's where you get to smack his big dumb head around. Again, wail away with YYYYY and YYYYB. You'll have to dodge a lot here as he shoots a bunch of different things at you, and this is where Selene's Light especially comes in handy. You may also want to have the bat form to help mitigate the damage and keep your combo counter high.
After finishing him off, you'll have at minimum 80k halos (and that's if you take a lot of hits and don't keep your combo counter up). Once you get the Gaze of Despair equipped and get better at dodging, you'll be hauling in tons of halos. Just now I popped him again and got almost 900k in a little over 6 minutes.
You could also farm him on higher difficulties but then you'd probably want to do YYYY-evade instead of going for the 5th hit and maybe equip something higher than Selene's Light, perhaps the Dark Rosary.
TL;DR VERSION = Chapter 7, equip Durga claws in lightning mode and Selene's Light & Gaze of Despair. Spam YYYYY combo. Halos get!
+edit+ after you're finished farming halos, you'll probably want to unequip Gaze of Despair unless you're a glutton for punishment.