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Towards the very end of the DLC, your objective will task you with exfiltrating from the prison by reaching the helipad you came in from. Before taking the elevator to the helipad, make a manual save.

Once you reach the helipad, you will confront both Mejia and the Fixer. Assuming you are stacking "Winners Don't Use Drugs (or Biocells)" on this playthrough, you will not have access to your CASIE aug and won't have the opportunity to convince Mejia to surrender. As a result, you are left with two options: side with Mejia and allow him to kill the Fixer or side with the Fixer and kill Mejia (this causes two nearby turrets to assault you and sometimes they kill the Fixer).

For the purposes of this achievement, you should choose to side with Mejia. Once the cutscene ends and he is about to execute the Fixer, quickly pull out either a Stun Gun or a Rifle and incapacitate/kill Mejia. With him unconscious/dead, you can now speak to the Fixer and decide his fate: leave him behind or bring him along back home.

The achievement will unlock once you have made your choice. If you chose to kill Mejia but were working on the "Code of Conduct" achievement, just reload your save and side with him this time, allowing him to execute the Fixer.