Partially MissableDepending on your choice at the end of the game it's possible to miss The Twins boss fight and see the end credits. After visiting the "Lighthouse" and falling unconscious you'll engage in a lengthy conversation with Granny Zina. During this conversation you'll be given two choices:
Option #1: Weak ending, game ends instantly and end credits roll.
Option #2: True ending, game continues, finishing with the Twins Boss fight.
Don't worry if you did select Option #1 just reload your previous save and complete the conversation with Granny Zina again.
The TwinsThe boss fight itself is pretty easy (even on Armageddon). Before entering the final room you'll have both a crafting station and a save point. You only have to defeat one of the two Twins to complete the boss fight. I recommend targeting "Left" who starts right in front of you. The best method (for me) was to fully upgrade the "Fat Boy" and pop a few consumables (Vodka/Milk) before opening the door. Hit "Left" with 4-5 direct impacts from the "Fat Boy" and it should kill her moving the boss fight to the final phase. In this final segment just do your best to avoid the red laser beams for 30 seconds until the boss fight ends. As the cutscene kicks in the achievement should pop.
See below for a video guide covering all of the above: