Tomb Raider I-III Remastered Starring Lara Croft

This achievement is unlocked by killing every enemies in Unfinished Business extension. There is only 2 levels with atlatean, so it shouldn't be too long. In level 1, you must use the shotgun for every encounter to make it easy, that's how the level was designed to be played. In level 2, it's the UZIs that should be the most used.

Here's the number of enemies per level:

Unfinished Business
Level 1 - Atlantean Stronghold: 31 kills
Level 2 - The Hive: 41 kills

You must complete the Hive level to unlock the achievement. Good luck!