This leveling trick is done in Map 26, when you re-fight Fienne after being forced to lose.

The hard method is for getting the most levels out of this trick, but the easy method will still give you plenty of levels (and GP) to steamroll the rest of the game.

Stuff required for the harder method will be italicized.

Gig Edicts (better equivalents will also work):

• Kick x1 (extras are optional)
  
• Switch x1 (x2)
  
• Bestow x1 (Can use Grant, but not recommended)
  
• Split x4
  
• Transform x1
  
• Fight (optional, about x4-6)
  
• Hold Hands x1

If you don't have the required Gig Edicts, you can refresh the shop by using Fight on them. Switch didn't show up for me until I was Shop Level 6, so if you can't find an item, try leveling your shop.

Decors:

• Cat's Meow x1
  
• Total Revolt x1
  
• Blessed March & Bonus Day x3

Don't equip Cat's Meow on any room, Once again, if you can't find it, try leveling up the shop and keep using Fight to refresh it.

Squad Setup

For the most part, the room type doesn't matter.

• Hero: Put the Hero in the back row and a single unit in front of it to tank hits. You can put extra units you'd like to level in this room if you'd like, but your Hero won't gain as much levels. Don't use a room with an extra permanent Decor (just pick a room using Home).
  
• Assist: 1 Pyremage in back row and 1 any unit in front of it to tank hits. Make sure the square the filler unit is on DOESN'T OVERLAP with the filler unit in your Hero's room. Don't add extra character you want to level in this room. Give this room Total Revolt and 1 extra decor that boosts the Pyremage's stats (do not use more than 2 decors as they will not transfer upon merging).
  
• Cleanup: Room (with Blitzkrieg) w/ Juno as leader and 3 Pyremages in back row. This squad's purpose is to use Only Hope on your Hero for bonus XP and kill Fienne when you're done. Give this room Decors to boost the Pyremages' stats. (Great Wisdom, Taboo Spell, Graceful Form works best).
  
• XP Farms: 3 units in the back row with units that can't attack from the back row, preferably ones weak to magic (Knights, Redflanks, etc.). Unsure if type of units affects XP gain. Give these rooms Blessed March and Bonus Day. Fill any remaining rooms with this setup (which should be 3).

Pre-battle:
- Kick the Potbellied Man in Astec so they hold a grudge. You need to do this so they spawn on the map on Turn 2.

The way this trick works is this: Use a Gig Edict in town to aggro a townsperson so that they spawn in as an enemy, Switch with them, use MAX Gigify to boost everyone except the Hero's level, wait for Switch to wear off, merge your Hero's squad to another one with the temporary level boost, then kill the lvl 2000+ enemy with your Hero's squad.

The most straightforward way to do this.

1. Summon your squads. Make sure the squad you're going to merge your Hero's squad to is within range.
2. Use Only Hope on your Hero with Juno's squad.
3. Wait for Turn 2. An enemy squad of 3 Knights will show up.
4. Use Switch on the Knight with your Hero.
5. Use MAX Gigify, giving everyone except your Hero bonus levels.
6. Give your Hero's squad Cat's Meow using Bestow or Grant. Just make sure no one is within attack range when it's your Hero's turn.
7. Wait for Switch to wear off, reverting controls.
8. Merge your Hero's squad to the Assist squad.
9. Kill the enemy unit with your Hero. Use Fight to increase damage. You want to kill the whole squad along with the leader to get the most XP. Use Kick or Hold Hands with another squad to get within range if needed.
10. Kill Fienne with the Cleanup squad.

This method requires more steps, but will give you even more XP. The basics are the same, except it involves splitting everything.

1. Summon your squads. Make sure the squad you're going to merge your Hero's squad to is within range.
2. Use Only Hope on your Hero with Juno's squad.
3. Use Separate to split all your XP Farm squads.
4. Wait for Turn 2. An enemy squad of 3 Knights will show up.
5. Split the enemy squad.
6. Use Switch on the Knights with your Hero and another squad.
6. Use MAX Gigify, giving everyone except your Hero bonus levels.
7. Give your Hero's squad Cat's Meow using Bestow or Grant. Just make sure no one is within attack range when it's your Hero's turn.
8. Wait for Switch to wear off, reverting controls.
9. Merge your Hero's squad to the Assist squad.
10. Kill every unit except the Cleanup squad with your Hero. Use Fight to increase damage. You want to kill the whole squad along with the leader to get the most XP. You can have your XP Farm squads get hit by Fienne to speed things up. Use Hold Hands and Kick with another squad on your Hero to move your Hero around.
11. Kill Fienne with the Cleanup squad.

If done properly, your Hero should be level 200+ (with no other units leveled).