Although the purpose of this guide is not to give a strategy on how to defeat the secret boss, keep in mind that some things will be useful to have before starting.

Firstly, the secret Megiddo, ???:???, is only available after having beaten the game at least once, so it will appear in a New Game Plus only, in the very first playthrough it won't appear at all.
Then, be sure to have a party level of 100* more or less, but more is better, and the assist's Illuminas "Varia" and "Invincible" (or the one of the girls' Love, which are all the same and are a stronger version of "Invicible") equipped on every character, or at least on the ones with whom you'll start the fight.

Furthermore, it should not be forgotten that, in battle, it is always possible to change team members, even the KOed ones, with the "MemberChange" command, the last one by scrolling down the action menu.
Also, to spice up the training, remember that, in the New Game Plus, it is possible to change the characters in their swimsuits, via the pink crystals, at the beginning of every training Megiddos.


* : for example, note that a level 156 Remy with all passives researched and with the strongest Illumina gets oneshotted by pretty much anything in the last training Megiddo at difficulty 5, so, without the assist Illuminas "Varia" and "Invincible" it is rather advised to not try to beat them in a regular way. However, we will see later that this is not essential to at least reach the secret boss, but it is to beat him instead 😊

Once you get to the Magna World for the third time, the pink and tentacled version of the planet of the Magnas, don't skip it with the New Game Plus's option and don't proceed further either.



Instead, head back to the Noah and enter it again, then go back to Earth from the main console, enter the simulator in the Hall of Bapthus, which will be available again, and face every Megiddos you're missing in the simulator at difficulty 1 (use the assist Illumina "Hide" and skip the bosses if you like), until you enter the last one, "Gaflet", then get to the boss, Azathoth, via the northwest shortcut through the wall.



Let the boss practically commit suicide with his Graviga magic attack against your "Varia" and then the tear to access the Secret Megiddo will open, at the end of which is the secret boss.

The ???:??? Megiddo and the secret boss are not affected by the difficulty chosen for training (but you're at difficulty 1 anyway 😉), so the dungeon will be a piece of cake compared to the previous ones at difficulty 5 (if you managed to grind a bit, you'll know what I mean), and also the boss is always the same, it hasn't much more HP compared to other lv5 ones, but the fight is tricker:

King of Magna
HP: 500,000
Resistances: Dark, Conceal
Weaknesses: None



As you may have understood, it is not necessary to finish the simulations on the 5th difficulty to unlock the boss, but only being in a New Game Plus is needed, however doing so is useful for leveling quickly.
The secret boss, however, is more annoying than strong, with its "Reaper" attack that kills a party member in one hit and cannot be prevented. Also, after he loses roughly half of his HP, he will change shape and in this phase he will attack two, sometimes three or even more, times per turn.