Baal: Complete Extra Map 9
Etna: Complete Extra Map 2
Flonne (Fallen Angel form): Complete Extra Map 2
Laharl: Complete Extra Map 4
Mid-Boss: Complete Extra Map 5
Myao: Complete Extra Map 8

Bonus maps:

Extra Map 1: Successfully complete the game
Extra Map 2: Complete Extra Map 1
Extra Map 3: Complete Extra Map 2
Extra Map 4: Complete Extra Map 3
Extra Map 5: Complete Extra Map 4
Extra Map 6: Complete a random dungeon with over 30 floors and enemies over level 300 on the bottom floor.
Extra Map 7: Complete Extra Map 1 through 6
Extra Map 8: Unlock Extra Map 6 and also complete a random dungeon with over 50 floors, and enemies over level 500 on the bottom floor.
Extra Map 9: Unlock Extra Map 6 and 8 and also complete a random dungeon with 99 floors, and enemies over level 1000 on the bottom floor.
Extra Map 10: Complete Extra Map 9

Easy experience:  Go to Phantom Isle, talk to Marona, and select "Create Character". Then, cycle through them all. If you see the title "Failure" or "Ruined", make that character. It does not matter what level it is. Note: If you do not see any of those titles, close out of that menu and try again. It may take a few minutes to find one. After creating said character with the "Failure" or "Ruined" title, pick him up and talk to Marona. She will ask if you want to banish him from the isle. Answer "Yes". The message "You have acquired the title "Failure/Ruined" will appear. Speak with your DungeonMonk and create a dungeon with enemies ridiculously higher than yours, preferably with few in number. Speak with your Titleist and scroll to the dungeons page. Put the title "Ruined" or "Failure" on the impossibly hard dungeon. The monsters in the dungeon will now be weakened dramatically. You will not get much experience in the battle, but afterwards in the post-battle screen, you will get a large amount of experience points.

Phantom Island: Kill your own teammates:  Walk up to your teammate and press R2 to attack them. Press Toward + R2 to grab and throw them. This is useful for easy experience.

Phantom Island: Unknown Phantom tricks:  Go to the Unknown Phantom at your home (Phantom Island) and attack him using R2 as stated in another hint. When you kill him, you obtain experience points and mana. To resurrect him, talk to Marona, go into create a character, then exit back out. The Unknown Phantom should be back at his original location, fully healed. Repeat this as desired. Note: If you pick up an object (an item or weapon) and hit him with it (for example, killing him and destroying his body afterwards), the item or weapon will gain mana. It also might gain experience. Also, if you kill the Unknown Phantom about twenty times, you will be able to make Whisps (type of monster).

Phantom Island: Change person you control:  To change which person you control on Phantom Island, pick up a Changebook. You can get one by jumping on top of the really big tree from Marona's house. Go to whomever you want to control and press R2 as if you were attacking them. You can use this with the "Unknown Phantom tricks" for getting experience or mana. This allows you to level up your Dungeon Monk, Merchant, Old Man, Old Granny, Healer, etc. without having to risk them in a battle. You can even control the Unknown Phantom itself, which means that if you level him up and let the others hit him after he has been leveled you can get even more experience or mana for your characters and items.

Phantom Island: Extra money:  You can find money around Phantom Island by jumping on certain places. You can find 100 Bordeaux by jumping in the stone circle in front of the house. There is also some behind the house at the bottom of the cliff in the water. There is also some on the roof of your house. Also, on the big tree next to the house you can find a book called the "Changebook". It is a very strong book for magic users. It is located on the edge where you can jump on to from your house"