You can get this achievement after completing the game, which kind of takes the pressure off don't you think?
I don't take credit for the solution although I can vouch for it, it is still exploitable in the Xbox One edition as shown in this guide: Solution for Deadeye in Tomb Raider
Go to Helicopter Hill. Re-establish the zip line as shown, and climb it. As soon as you see the baddies, jump back on the line and after dropping off, get ready to do some shooting! As long as you don't kill these guys on the platform (where they spawn and come at you from), but rather after they jump on the line, rinse and repeat as they spawn in twos!
No second playthru required!
EDIT: Please read PiCkLE SQU1D's solution below if you are having trouble.
This info is included in the comments section and I should have updated my solution. Sorry!
Unfortunately, if you kill the enemies - on the platform - before completing the game, they will not
respawn. The only solution is to start a new game. Sorry.
To clarify, if you kill the enemy while they are on the platform
- they will not
respawn. If you go to the platform, and let them follow you off of the platform - its a turkey shoot. Shoot them on the line or after they drop - no penalty. They will respawn every time you come to this area and follow the instructions to get them to respawn. However, if they die on the original platform anytime during the game
, they will not respawn.
My tactic was to go to the platform the enemy spawns from, deploy and hop onto the zip line to the upper platform, turn around and hide. Shoot the enemy AFTER they had jumped up onto the line, never while they are on the platform.