One Who Has Proven Their Power achievement requrements guide (NG+ ONLY)

This short guide explains how to unlock a secret boss fight on your NG+ playthrough. Beating this boss unlocks One Who Has Proven Their Power achievement. This secret boss is way harder to unlock than the ones in P3 and P5, but it will be pretty simple if you don't forget to fulfill all of the requirements for it.

Keep in mind that this boss can only be fought if your previous playthrough's clear data was recorded after you have cleared the game with the Golden Ending. I explained how to get the Golden Ending here.

You can't fight this boss on your first playthrough, so this guide implies that you have already cleared the game once and aren't afraid of spoilers. I will still try to keep the spoilers as minor as possible.

Fulfill of the requirements to reach the Golden Ending of the game once again on this playthrough.

After you clear a TV World dungeon and move on with the story you can return to it to find another boss at the same place where you have beaten the first one (on the final floor of each dungeon). Every dungeon you cleared has it.
This boss is more challenging than the first one each dungeon has, but since you'll be on NG+ it shouldn't be much of a problem.

You must beat every single second boss before you finish Magatsu Inaba dungeon.

Max Margaret's S. Link before you finish Magatsu Inaba dungeon as well. This entire S. Link can be done as fast as a single day if you have the personas Margaret asks you to fuse.

Make a savefile during Magatsu Inaba dungeon and another one before doing anything on 03/20. This way you won't have to start your playthrough all over again if you mess up.

On this date after you meet Izanami, do not enter the TV World. This is extremely important.

Instead go into the Velvet Room and recieve an Invitation Card from Margaret. You won't recieve it if you visit the Velvet Room after you enter the TV World. If it doesn't happen then it must mean that you have messed something up, so go and reload one of the savefiles I told you to make before. This would be still better than beating the game again from the beginning.

After you do so you are now free to enter the TV World. But after you do so, do not challenge Izanami now. Prepare your best personas and party members, and instead go to the top of the Heaven dungeon to fight the secret boss! This is the hardest boss in the game, so be as ready as you possibly can. Beating it will unlock the One Who Has Proven Their Power achievement.