Atomic Heart

*Comments are saying this has been patched, this will now only work if you play an unpatched version of the game,

Credit to REA AONO on the forums.

After beating the game on ANY difficulty return to the main menu and start a New Game selecting Armageddon difficulty. Upon getting into the game, pause and load your save from before fighting the twins.
Defeat the twins and the achievement will pop.

Been trying other methods for days and finally got it to pop waiting till you get out of the boat before loading to granny's house. This is what worked for me and builds off comments from @iitsStatic and @lB r o IL y I in @GMERSCORE JUNKY's solution.

If anyone is having trouble:
-Beat game on any difficulty (I did this weeks ago and am even missing half the story achievments at time of getting this achievment)
-Quit game
-Restart console
-Start a new game on Armageddon
-Let the opening scene play until you get out of the boat and get an auto save
-Load a save to Granny's house before twins fight (A lot of people say to load straight to the final twins fight, but I think the first time I played through I had died at the quick time event and that might be messing some people up)
-Beat both twin boss fights without dying (recommend quitting game and restart the process from there if you die, even in the quick time events).
-Should pop after beating twins and they explode, had to wait like 15 seconds for mine to pop

Tips for killing twins:
-Use rail gun and fat boy
-Bring as many rockets as you have
-Bring 2x dynamo (recharges energy significantly faster) and 2x condensed milk (increases damage), use one of each at the start of each fight.
-Bring lots of large and medium heals
Use the rail gun and shok as much as possible. Use fat boy when energy is recharging. Should be a piece of cake. Hardest part is the quick time events.
Hope this helps those struggling!

Not sure if this will help anyone but it worked for me. I didn't need to start a new game on Armageddon and play another save or anything. All I did was load the save I beat the game on (for me it was on the easiest difficulty) right before the twins boss fight (Chelomey: High-Rise Final Floor), go into OPTIONS, tab over once to GAMEPLAY, and the fourth (4th) setting is game difficulty. I switched mine from "Peaceful Atom" to "Armageddon" and simply went on to beat the twins. The achievement popped for me as soon as they died. Below is the loadout I used and was able to beat them first try with although I ran out of heals right at the end so I would recommend subbing out the medium capsules with more large ones. If you do not need the loadout, I hope this helps a little bit for those still struggling to find a way to get the achievement without doing the entire game on the hardest difficulty!


Loadout:

KALASH -
level 4 conventional stock
level 4 electromagnetic polarizer barrel
level 4 cartridge module (I used one (1) electric cartridge because I forgot to put in more in my gun during the fight so bring about 3 or 4 and remember to use them when you run out!)
Basic magazine
Collimator sight

ELECTRO -
***THIS GUN WAS COMPLETELY STOCK EXCEPT THE EMP GENERATOR***
All I needed it for was to hold down the left trigger (LT) to release a pulse that did a fair bit of damage so I would HIGHLY RECCOMEND upgrading this gun if you can as it will help you out tremendously. If you cannot afford to fully upgrade it, try to upgrade the emp generator stunning head as much as you can. ALSO, if you're going to be relying on this gun, bring LOTS of electro and dynamo as they both will help with your energy recovery. This is an absolute must if you're going to be using the electros' charge ability since I was only able to get two charges out of one full bar of energy. One more thing, I DID NOT have ANY of the energy management upgrades either so if you have these I'm sure they will help you out a ton!

CONSUMABLES -
six (6) large neuromed capsule
three (3) medium neuromed capsule
one (1) small neuromed capsule (right before the fight to get my health to full)
I did not use any electro or dynamo as I previously mentioned but I would definitely recommend if you have the space.
four (4) condensed milk - (increasing damage)
one (1) vodka - (significantly reduces incoming damage)
NOTE: You can but the milk and vodka in your weapon wheel by holding your reload button to open your weapon wheel, hovering over an empty slot (or any slot you prefer) and selecting it with your fire button (for me it is RT). Once there, simply hover over the desired consumable and install it. ***PLEASE NOTE THAT YOU MUST HOVER OVER IT AND HIT RT AGAIN AS SIMPLY RELEASING THE RELOAD BUTTON WILL CAUSE YOU TO USE THE ITEM YOU WERE HOVERING OVER AND ALSO NOT PUT IT IN YOUR WHEEL***

AMMO -
Since I only used the kalash and electro, the only ammo I needed was for the kalash and I carried seven (7) stacks of 90 equaling a total of 630 rounds which was a bit overkill but better safe than sorry.

PERKS & SKILLS -
- I had a fully upgraded character
- I had an almost fully upgraded SHOK and used it about 2 or 3 times throughout the fight in total. I would totally recommend using it as much as possible to keep your dps up while your two skills recharge!
- I used FROSTBITE with only the horizontal row of upgrades for it and only used it about twice, however, I noticed it did help stop them in their place so you can keep the damage on them but it did not apply a frozen effect to them.
- Finally I used a fully upgraded MASS TELEKINESIS except for "energy vampire (226 neuropolymer)" This skill is INCREDIBLE against the twins as it will not only keep them in the air for you too shoot them with your other hand and negate any and all attacks they were going to do, but it will also deal a significant amount of damage when you slam them to the ground! I suggest using this one the most while also cycling your other skill with UP on the D-Pad and using SHOK as well.

If you read this far, you're a legend and I hope this helps a little bit in some way shape or form!

Not sure if this will help anyone but it worked for me. I didn't need to start a new game on Armageddon and play another save or anything. All I did was load the save I beat the game on (for me it was on the easiest difficulty) right before the twins boss fight (Chelomey: High-Rise Final Floor), go into OPTIONS, tab over once to GAMEPLAY, and the fourth (4th) setting is game difficulty. I switched mine from "Peaceful Atom" to "Armageddon" and simply went on to beat the twins. The achievement popped for me as soon as they died. Below is the loadout I used and was able to beat them first try with although I ran out of heals right at the end so I would recommend subbing out the medium capsules with more large ones. If you do not need the loadout, I hope this helps a little bit for those still struggling to find a way to get the achievement without doing the entire game on the hardest difficulty!


Loadout:

KALASH -
level 4 conventional stock
level 4 electromagnetic polarizer barrel
level 4 cartridge module (I used one (1) electric cartridge because I forgot to put in more in my gun during the fight so bring about 3 or 4 and remember to use them when you run out!)
Basic magazine
Collimator sight

ELECTRO -
***THIS GUN WAS COMPLETELY STOCK EXCEPT THE EMP GENERATOR***
All I needed it for was to hold down the left trigger (LT) to release a pulse that did a fair bit of damage so I would HIGHLY RECCOMEND upgrading this gun if you can as it will help you out tremendously. If you cannot afford to fully upgrade it, try to upgrade the emp generator stunning head as much as you can. ALSO, if you're going to be relying on this gun, bring LOTS of electro and dynamo as they both will help with your energy recovery. This is an absolute must if you're going to be using the electros' charge ability since I was only able to get two charges out of one full bar of energy. One more thing, I DID NOT have ANY of the energy management upgrades either so if you have these I'm sure they will help you out a ton!

CONSUMABLES -
six (6) large neuromed capsule
three (3) medium neuromed capsule
one (1) small neuromed capsule (right before the fight to get my health to full)
I did not use any electro or dynamo as I previously mentioned but I would definitely recommend if you have the space.
four (4) condensed milk - (increasing damage)
one (1) vodka - (significantly reduces incoming damage)
NOTE: You can but the milk and vodka in your weapon wheel by holding your reload button to open your weapon wheel, hovering over an empty slot (or any slot you prefer) and selecting it with your fire button (for me it is RT). Once there, simply hover over the desired consumable and install it. ***PLEASE NOTE THAT YOU MUST HOVER OVER IT AND HIT RT AGAIN AS SIMPLY RELEASING THE RELOAD BUTTON WILL CAUSE YOU TO USE THE ITEM YOU WERE HOVERING OVER AND ALSO NOT PUT IT IN YOUR WHEEL***

AMMO -
Since I only used the kalash and electro, the only ammo I needed was for the kalash and I carried seven (7) stacks of 90 equaling a total of 630 rounds which was a bit overkill but better safe than sorry.

PERKS & SKILLS -
- I had a fully upgraded character
- I had an almost fully upgraded SHOK and used it about 2 or 3 times throughout the fight in total. I would totally recommend using it as much as possible to keep your dps up while your two skills recharge!
- I used FROSTBITE with only the horizontal row of upgrades for it and only used it about twice, however, I noticed it did help stop them in their place so you can keep the damage on them but it did not apply a frozen effect to them.
- Finally I used a fully upgraded MASS TELEKINESIS except for "energy vampire (226 neuropolymer)" This skill is INCREDIBLE against the twins as it will not only keep them in the air for you too shoot them with your other hand and negate any and all attacks they were going to do, but it will also deal a significant amount of damage when you slam them to the ground! I suggest using this one the most while also cycling your other skill with UP on the D-Pad and using SHOK as well.

If you read this far, you're a legend and I hope this helps a little bit in some way shape or form!

The cheese strat still works as of 26/11/2023:

1. Make a save at final door to both Twins fight
2. Beat the game
3. Start New Game on Armageddon
4. Play up until you get off the boat, you want that auto save to appear
5. Reload your save from the door to the Twins fight
6. Pass QTEs and the fight itself. I used the Fatboy with homing missile mod and had 80 rockets at hand with 8 medium heals
7. Soon as they do their death spin animation, it should pop
8. Retry from step 3 if not unlocking

If this was your last cheevo towards the “100% achievements earned” achievement and your counter is stuck at 97% like it was for myself. Power cycle the console and re-open the application. Start New Game+ and it should pop immediately after selecting a difficulty and starting the new playthrough.

If anyone else played through the entire game on Armageddon (never changing the difficulty) and the achievement didn't unlock at the end, here's something that worked for me:

1. Exit the game and reopen it
2. Load a save from before the fight with the Twins, but NOT the auto save, load a manual one (I loaded the one from right outside the final room)
3. Kill the Twins without failing the QTEs / dying

I tried redoing the fight from the auto save and the achievement didn't unlock. It was only when I did it from the manual save that the achievement unlocked immediately after I killed the Twins. I think the reason why it didn't work from the auto save is that I had failed the QTEs and died, so maybe that broke the auto save, who knows.

I found a new way first delete data then play new game starting with armageddon
1. When you see the enemy, start, make the difficulty easy, when you kill them, change the difficulty to armageddon

2. Before the save game, turn the difficulty setting to armageddon

3. When the boss fight comes, fight him on easy difficulty. When he is low on blood, change the difficulty to armageddon, then finish