EDIT: Video is up. Read to bottom to view it.
EDIT 2: As of 3/28, I have found the minimum requirements for the true ending. The solution has been updated to reflect this.
For this achievement you'll need the following done before completing the main quest Mother Claudia:
1) Reach Rank 4 Affinity 75% with all characters original to Fatal Bullet. (Your ArFA-sys, Kureha, Zeliska, Itsuki, and Bazalt Joe)
2) Reach Rank 2 Affinity with the remainder of the cast. (This does not include LLENN, Fukaziroh, Pitohui, M, or any additional characters from the incoming expansions.)
3) View all events for each character while on your avatar. (They'll appear as green word bubbles in the Urban District of the SBC Glocken, this includes the sleepover scenes for your ArFA-sys, Kureha, Zeliska, Itsuki, and Bazalt Joe.)
Note that you'll earn these achievements as you do this:
The Proof of Friendship achievement in Sword Art Online: Fatal Bullet worth 76 pointsGrow close to Kirito, Asuna, Leafa, Sinon, Lisbeth, Silica, Agil, Klein, Yuuki, Yui, and Argo.
Note while you don't have to reach Rank 4 Affinity with everyone, you can earn this achievement if you do (as well as their dating/sleepover scenes for both your avatar and Kirito):
You do NOT need to complete Kirito mode in order to get the true ending of the game, but doing so can net you these achievements if you do:
And this if you have every Weapon Art:
As well as this if you complete everyone's events as Kirito (who are found in the Lobby of the SBC Glocken):
Once you've met the requirements, return to your room and your ArFA-sys will give you an "ArFA-sys Charm", one of the keys to the true ending of the game. Once you've got that, proceed through the main story of the game. When you get to the point where you choose to save either Kureha or Zeliska, choose whoever, you'll be saving both. If you've followed this to the book, the final main quest will be changed from "Clincher" to "Fatal Bullet", hence the subtitle of the game. The quest will offer another objective aside from escaping the death game which is the next step to achieve the true ending.
To save both Kureha and Zeliska from certain death, you MUST put massive DPS onto the GM (the normal final boss) when it charges up it's most powerful attack. Fail to do so and you'll get the normal ending instead. It will attempt this at 40% health and if you put enough DPS on the boss, it'll be stunned. Use this opportunity to finish it off. If it gets back up, it'll try again at 15%. Defeat the boss to save Kureha and Zeliska. Then sit back for a bit and watch as the cinematic unfolds.
Now for the last part. Defeat the true final boss of the main game, Dark Judgement. Its level will be five levels over the GM. 85 on Easy/Normal and 155 on Extreme difficulty (five levels over the game's original cap, 150). Defeat the true final boss of the game and you'll finally be in the clear. Watch the cut scenes as they reveal the true ending to Fatal Bullet (which I thought it was awesome). Once you've select either option to restart the story from the beginning (I selected "No" as I wanted to grind for some Rank 7 weapons), you'll earn the achievement.
Note: You do NOT have to do it on Extreme difficulty to earn the true ending of the game, but it is recommended as you'll earn other achievements efficiently as you do.
Here is the final boss battle: