Remember that time I said last update was going to be the "last" one?
Well, guess I was wrong.

Some things were pointed out to me some bugs regarding the last secret boss, so I made adjustments to it. Also, I also totally forgot to tell you guys how to access the secret boss! Who may they be?!!!
There are some small spoilers regarding the appearance of some characters on the game, but nothing overall the whole original story of the game, so rest assure! This will help you do as much as possible in your first run, without having to start your game from zero again just for it

Following, the order of actions to perform:

1. After you save T-chan, talk to her and beat the Quiz. After that, talk to her again and she will give you the Broken Black Horn (Fig. 1)
2. Talk to AO-chan in the Crystal Room and she will mention that Irys now can take you to a new location inside the Void
3. Once you reach the new Void area (called "Unknown"), go to the top side of the area, where the weird purple ground becomes green and grassy. Search for the section in which you can travel to another area
4. You will meet a really familiar character here.... after some dialogue, you will be able to enter to the ruins in front of you, but......
5. Now you will be able to play as Irys! (Fig. 2) Go to the same building you went before and this time you will be able to proceed. There are a series of puzzles to open a door to proceed. The puzzles are not that hard to clear, so I don't think there is need to explain them here
6. Going through the door and a stupidly long corridor, you will meet a more or less difficult boss.... After defeating it, you will receive the weapon Kurenai
7. The last room of this area is a mysterious library.... aside of the fancy chair you find the Ancient Diary (Fig. 3)
8. Go back outside the ruins and you will be teleported to the Crystal Room again. Talk to AO-chan and then you will have to return all the way back to the library (I know, at this point you probably hate me, but those are the rules of RPG games)
9. This time, the library will have a visitor, which will take the Diary from you.  At this point, you did the hardest part of all of this, so now you can continue with the game normally
10. On the room before the final fight (when all the girls are reunited), you will have to talk to A-chan, Enma and then on the highest part of the area, a brilliant crystal should have appeared. Interacting with it, you will be teleported again to the room before the library. Be well prepared, because once you enter the library, a boss fight will start!
11. Once you beat that boss, now all that is left is to beat the final boss of the game and, after the credits, you will access an extra portion of the game. Here you will be able to prepare for the last battle against this boss
12. Defeat the boss and brag about it here! 

That's basically the guide to reach the final boss. You will be able to access a new section in Fantasy Battle Mode to fight against these hidden bosses as you defeat them in the story mode.
Well, that's it, hope you have enjoyed this game this far