I wrote up a quick post-game roadmap and levelling guide. It's kind of short because I'm not very good at explaining things :S
Any feedback or additions would be greatly appreciated.
1. Map 7-3 Corpse Eaters
First of all, you'll want everyone to have a special that hits a 3x3 area, namely Big Bang or a magic spell. Characters can learn from other characters by absorbing (in the Homeroom), or by going through the Class World.
Pass three stronger enemies bills so that the enemies are level 99 on this map. Grind here until you can finish the House of Ordeals.
2. House of Ordeals
Pass the bill in the Homeroom to unlock the House of Ordeals. HoO3 is a huge pain in the ass, but once you get through it, you're golden. HoO4 is absolutely the best grinding spot in the game.
Enemies in HoO4 are normally level 400. As you get stronger, you can pass stronger enemies bills to speed up grinding.
3. Extra Maps
Talk to an NPC in the academy, then pass the bill in the Homeroom to unlock the first extra map. Repeat the process and try to finish the extra map 4, which features Asagi at level 500.
4. Raspberyl Version
After defeating Asagi, enemies in Raspberyl mode will start to equip trapezohedrons. Steal one! If you're aiming for a specific rarity, in map 1-4 just throw your thief to the target enemy and check her trap's rarity. If it's not what you want, quit reload and try again.
You can finish Raspberyl mode now if you want, or just head back to Mao mode. Main thing here is to steal at least one trapezohedron.
5. Diez Gentlemen sidequest
The sidequest becomes available after finishing Extra Map 1. Again, these maps have to be unlocked by passing bills.
The Diez Gentlemen sidequest culminates in a battle with a level 650 and a level 700. Your prize is a legendary puppy paw stick! The PPS has a 20% chance of stealing an item upon defeating an enemy.
6. Pirates, statisticians, and puppy paw stick
Go through the item world and defeat a group of pirates to unlock the PPP club. Pirates begin appearing on floor 21+ and become more frequent at higher floors, so if you have a legendary item, use that.
With the PPP club, you'll be able to reverse pirate. When reverse pirating, you'll find lots of useful innocents, namely statisticians. There's a limit to the number of times an item can be reverse pirated, so if you don't see a statistician, quit reload and try again.
After capturing and subduing a statistician, you'll be able to move it from item to item. Put it in your favorite item, then equip it to one of your favorite characters. Make sure that character has nothing else equipped.
Go into that character's class world. When you encounter a dropout, defeat the dropout using the puppy paw stick. You can use both normal or special attacks. Either way it'll give you a 20% chance of stealing the item that holds the statistician. You can't steal from dropouts using hands, but the PPS is magic!
Transfer and combine the statisticians to level it up. Its max level is 300. Keep repeating the item duplication process until you have numerous stacks of statisticians.
So what does a statistician do? Gives you X% more experience, where X is the level of the statistician. So a single level 300 statistician will quadruple your EXP gain.
Once you're tired of item duping, stuff you equipment full of statistics, and head back to HoO4. Now watch your levels fly!
7. X-Dimension levels
To unlock the X-Dimension levels, you need to collect illegal tickets. In the item world, tickets can be taken from pirate captains by defeating them. Also in the item world, there are two mystery rooms that sell tickets; one appears on floors 1-50, one appears on floors 51-100.
8. Item/character maximization
If you haven't maxed out your trapezohedron yet, now would be a good time. Also spend some time reverse pirating, collecting useful innocents, item duping, etc. Refer to the item/character maximization guide for more.
And feel free to finish up the Extra Maps, the Parallel Worlder's side stories, and whatever else you want to do. The only thing left is...
9. Land of Carnage
To unlock the Land of Carnage, finish at least 25 of the X-Dimension levels.
Here you'll find the best items in the game. If you've got some maxed out trapezohedrons and stats in the millions range, you shouldn't have a problem handling the Land of Carnage. Don't forget to visit this land's HoO4. You'll be able to level up even faster!
To get a rank 40 weapon, steal it from the Item God on the 100th floor of a rank 39 weapon.
To get a rank 41 weapon, steal it from the Item God on the 100th floor of a rank 40 weapon.
