The EX maps are not too complicated to get, it is mostly just tedious.

To start, you will need to beat the original game, there will be a letter in your mail box that will talk about a Stamp rally.
The locations will go as follows:
Mount Kazan - Hill of death.
Terra Firma - Barren Earth
Frigidia - Brave's - Seal
Desert Hill - A Test of Will.

Once you have finished the Stamp rally you can start on the Extra map 1 which is located on Verdant Guardiana when you click on it it should have an uncleared level that you can start.

After beating EX map 1 on Verdant Guardiana, the second one should be unlocked and located on a new location that is in the upper left corner called
"Overlord's Castle"

There should be 4 levels in total in the Overlord's Castle, respectively going from EX maps 2-5 once you've finished them.
you will need to clear a random level 30+ dungeon at the dungeon monk, which needs to have level 300+ enemies at the last floor. to unlock EX map 6,

Once EX map 6 is beaten, you can jump straight into EX map 7.
afterwords you will need to complete another random dungeon but this time you will need to complete one that has 50+ floors and ending with level 500+ enemies at the end. once you collect the title from the dungeon you will be able to fight the next EX map 8

Once beaten you will yet again need to go talk to the dungeon monk and clear a level 99 dungeon that needs to end with level 1000+ enemies, which should unlock EX map 9. Honestly this should be easy for you if you are wanting to take on the next HUGE level jump of EX map 10

After clearing the Dungeons and the 9 EX maps. EX map 10 will unlock at the tower on the right of the map, This fight is brutal if you are not prepared, there is a link below from Carbunkle Flux, that will help you get everything ready for this fight,

https://gamefaqs.gamespot.com/ps2/919034-phantom-brave/faqs/37762

after triumphing EX map 10, the last two will be right above EX map 10. EX map 11 will be in the middle of the tower and EX 12 will be at the top of the tower. Again these fights are a HUGE level and difficulty jump that you will need to prepare for. but once you have finished fighting the insane space looking penguins that you will have beaten the hardest thing the game can throw at you!

I hope this helps!

This is the first time making a guild, so please don't be to hard on me haha, if anything is confusing let me know! I will clear it up as soon as i can!