This achievement is unlocked for getting all 26 game endings--the five main story endings A through E and the 21 'bad' endings F through Z. When you get a game over ending, you will see that the save file that you have been playing will have the letter corresponding to the ending. Note that the DLC 3C3C1D119440927 ending depicted by a triangle icon is not required for this achievement--only the 26 base game endings each corresponding to a letter of the alphabet.
The easiest way to get this achievement is to buy it from the special achievement shop available after your third playthrough. It costs 200,000G. It is achievement number 46. The endings, themselves, do not take much time. However, the prerequisite for one of the endings, the Y ending, requires you to find all weapons in the game and fully upgrade them. If you are not prepared to undertake that time consuming endeavor, your best option is to buy this achievement for 200,000G.
If you are going to unlock this achievement in game play, there is one
missable ending. There is a special boss fight that is available when you upgrade all 39 weapons in the game to the max. If you look at the map, you will see there is a quest marker in the commercial facility. Go there to initiate the quest. As you follow the quest, you will be challenged to a special boss encounter. One of the game over endings is to survive the initial onslaught but lose the final segment by failing to destroy the boss before the counter expires. The reason it is missable is that if you beat the boss without triggering this ending, the battle cannot be repeated unless you load a save that was created before the encounter in which case you would lose your game progress that would include, at a minimum, the boss's defeat.
If you want to get this achievement through normal game play, an efficient approach is as follows. Complete the first three playthroughs. During your playthroughs, you may get as many of the 'bad' game over endings that you want and it is possible to get all 21 'bad' endings during the first three playthroughs. The important thing is that you will get the A and B story endings and either the C or D ending depending on your choice. You can then use chapter select to get the remaining bad endings including all 21 if you got no bad endings during normal play. Once you have endings F through Z, you can complete the game by using chapter select and choosing Chapter 17-09: 9S and A2. Choose the character you did not select in your third playthrough. When you finish the chapter, you will get either the C or D ending. If you exit from the top down shooting game that ensues by answering 'Yes' to any one of several questions that ask you if you want to give up, you will go back to the title screen and the achievement will unlock for the C or D ending depending on your choice. If you continue playing, all ending related achievements will unlock sequentially at the end of the game C or D depending on your choice, E, and this achievement. If you have all other achievements, you will also unlock the Final Words achievement for unlocking the other 47 achievements.
Caution: There is no right sequence or wrong sequence to obtain the endings. However, if you obtain the E ending without unlocking all the other achievements (excepting Final Words), you should proceed with care. After the achievements unlock, you are asked if you would like to help other players. If you answer 'yes', you will delete all your data. You will see it disappearing from your screen bit by bit--all your saves, weapons, quests, items, data, and so on. You are given several opportunities to change your mind and answer 'no'. So, this is not something you might do by accident. Just make sure if you want to play further in your current game, you must answer 'no'.
The story endings are obtained through normal game play. Since I am writing a separate guide for each one, I shall not list them here. The following, in spoiler marks, lists all bad endings. They can all be obtained during regular game play. However, each one is annotated so you can use chapter select after your third playthrough and obtain it then. The annotation depicts, chapter, section, and playthrough following an underscore if applicable. For example, 01-01_1 means Chapter 01, section 01, first playthrough
*** Spoiler - click to reveal ***F Chapter 01-03_2: Mission Failed - When 9S tries to restore his OS for the first time, let the timer expire
G Chapter 01-01_2: HunGry for Knowledge - Leave the mission area when the first time you control 9S
H Chapter 05-03: A Mountain too HigH – Leave the mission area the second time Goliath appears
I Chapter 08-01_2: No I in Team – As 2B, after Defeating Adam Copied City, abandon 9S.
J Chapter 09-02_3: Bad Judgement - Kill the religious leader in the Abandoned Factory upon meeting it.
K Any chapter where you have a mackerel: Aji Wo Kutta - Eat the mackerel given by Jackass or eat one that you have obtained by fishing.
L Chapter 10-02: Lone Wolf - Abandon the mission area in the final battle of Route A or B. The time to do it is just before you defeat the spherical enemy. Afterwards, the game prevents you from leaving the mission area.
M Chapter 14-03: Break TiMe - Choose to not help Pascal's Village near the end of Route C
N Chapter 07-02: No Man's Village - Kill all machines in Pascal's Village
O Chapter 11-04: Just YOu and Me - Abandon the mission area at the start of Route C
P Chapter 11-06: CorruPtion – In Route C, as 2B, do not go to the commercial facility & let timer expire
Q Chapter 11-07: Questionable Actions – As 9S in Route C, leave the mission area without helping 2B
R Chapter 14-05: MaveRick – As A2, attack any villager or Pascal inside the Abandoned Factory
S Chapter 17-01: City EScape – Leave the mission area when Devola and Popola come to help
T Chapter (Any chapter where you have access to your chips): FaTal Error – Sell or remove the OS plug-in chip
U Chapter (Any chapter where you can go to the bunker): DebUnked – Self-destruct inside the Bunker.
V Chapter 17-01: Wreckless BraVery – Fight enemies with Popola and Devola instead of hacking the tower
W Chapter 01-01_1: Broken Wings – As A2, die during the flying sequence in the prologue
X Chapter 11-06: Time to RelaX – In route C, when you first control A2, leave the mission area
Y Any chapter where you have all weapons upgraded and can go to the commercial facility—can be done by any character alone. Doing it in a chapter with 2B in the lead and 9S assisting is the easiest: HeadY Battle – Survive the secret boss long enough for the timer to start but let it expire before destroying it
Z Chapter (14-01): OverZealous – As A2, kill Pascal after she helps him when they first meet