You must have:

• Access to item world
  
• Several Mr. Gency Exit items (more is better, suggest 5-8)
  
• Enough mana to pass 110 mana worth of bills in the Strategy Assembly per Mr. Gency Exit item

It will help if you have any of the following too (these are optional):

• Access to level 3-4 (Poisondise / Hazardous Highway 4) (optional for farming up captures/citizens if needed for first 'high' level item)
  
• Any higher level items already (example for me, items from martial training level 1)
  
• "Filler" items for levelling up bonus gauge (Filler 0.5, Filler Up, Filler G, etc)

I've seen guides/videos where 'low level' meant 'beat the game and have farmed for 20 hours after that', so to clarify:

• By "low level" I mean: as early as you can access Item World, and capture 50 NPCs.
  
• By "high level" I mean: the level of items you would get on the bonus gauge in item worlds containing NPCs of level ~800.

Credit to youtube user Juan Hernandez, I don't know if they use steam or not. I was looking for a money farm and found their video which I've put into text form and added to a bit to make this guide https://www.youtube.com/watch?v=tRNKRaH4Pps

If you don't have any high level items that's ok you don't need any to start! Magic!

You'll just need to do the Boost Pop, Boost Pop More and Boost Pop Even More quests from quest giver. Result of this will be an Exp Potion IV. Don't drink it!

To complete these quests you will need to capture 50 prisoners and make them citizens. Easiest way to do this is to use Capture 2 skill in level 3-4 "Hazardous Highway 4" to capture 3 NPCs at once until you have enough. It goes fast.

(if you don't have Capture 2, you need to put a/some members into the Capture Squad at the Squad NPC in your pocket netherworld, then capture some NPCs, interrogate them until they have no SP left, then use them to level up the Capture Squad, one of the perks of a higher level squad is the Capture 2 skill)



.
.
Go to the interrogation NPC in pocket netherworld and make 50 prisoners into citizens then go complete your quests. You'll receive Exp Potion I-IV, you want to make sure to hang onto Exp Potion IV.


.
.

Set cheat shop enemy strength to 20 stars.


.
.
Go to Strategy Assembly and pass the two bills "Better Bonuses!" and "Charge Bonus Gauge!"

This sets your bonus gauge to level 3, and gives you better items.




.
.

Go to the Item World NPC now, choose the item which gives you the highest level NPCs in the item world. You won't be fighting any, but their level affects bonus gauge rewards and that's what you care about.

The Item World for Exp Potion IV will give you NPC levels of 225 but you may have items which give higher level NPCs.


.
.
Once in the item world you'll need to check a few things, and exit/retry if they aren't in your favor:

• Exit can't have an NPC standing on it, you won't be able to kill them (you could lift/throw, but easier to retry level, up to you (editedit: turns out you can't lift an enemy on the exit square, I'd never tried))
  
• Check bonus rewards, you'll be getting the first 3 rewards at minimum, but using up a Mr. Gency Exit so make sure you're getting at least one good item. Higher item level is better (you can safely exit the level and retry it without losing anything, to get a sense of what sorts of items you CAN get and going for one of the better ones).

.
.
Move one character near the exit square, throw them into the exit using another character. On the next level just move one character a square and use a Mr. Gency Exit item. Enjoy your new higher level loot!


.
.
Repeat this process using the higher level loot you just gained, it will have higher level NPCs in it, for better loot in that item using this method (to a point, it doesn't get better forever)


.
.

Once you've done this a few times to get items way above your level (it seemed to top out around NPC level 810 for me), if you have any "Filler" items for raising the bonus gauge in your inventory it's a good time to use them. Retry an item world level until there are items from level 1-8 on the bonus gauge that will help you. Before throwing your character onto the exit, bring out several others and have them use the Filler items to get to bonus gauge level 8, then throw character into exit. (Level 9 would be pointless cause you don't need the bonus item level)

To get more Mr. Gency Exits you can farm the Gate Key (super low level enemies) 10 levels at a time quite easily, or any low level item to 10 and beat the boss. You don't have to kill everything on levels 1-9 you just have to get to the exits and then beat boss on level 10.

You can sell any unneeded high level items from doing this for a lot of HL if you choose.

Here's me on round 3 of doing this, the NPC levels have already gone from 225 to 810. I retried until I had a bonus gauge full of useful items, levelled it up then cleared the level. (it seems to have maxed out around that NPC level 810 but I only tried 2 more times after this)


Enjoy!