This video covers fighting the stronger Sin version of the final boss, and one of the extra wishes you can make. Here are the timestamps and notes.
0:00 Vanitas informs the Main Character he can get an extra wish
1:05 Yog-Sothoth shows up
2:10 Final battle against the Sin Monark's
2:11 Sin Monarks fight
14:03 Extra wish
16:12 Extra dialogue with Nozomi and Vanitas
16:59 New scene in the Infirmary
17:39 New dialogue from Neiko
Notes: First, in order to fight the Sin version of the final boss, you need to fight the Sin version of every Monark. The game wants you to grind out Abyssal fights to get the numbers for the Sin fight. Do Not grind out the Abyss fights, instead just look up the numbers to the Sin fights (the game even encourages people to share numbers) and just skip the grind because the drop rate is low. Once you beat all 9 Sin Monarks, a new number will appear on the phone in the same place that you found the number for the final boss. The Sin number will be different then the old final boss number.
Second, you will only be able to use the Main Character and the Fiends for the fight. Make sure that the ones you use are max level, since the Sin Monarks are all max level.
Third, about the choices of the final wish. The first two wishes (Kakeru and Sora choices) are the only ones that give any extra scene/dialogue. The other choices (a ton of spirits and max ego) are pointless since when you are at Max level you already probably have max ego and spirit is pointless.
Finally, you only get 1 extra wish. If you want to see the rest of the choices, make sure to save before you do this fight, then do the fight, make the first choice, then load it the previous save, fight the boss again, and then make the other choice.
There is also no new game plus as of making this video, so this is the final thing to do in the game.
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