To unlock this achievement you need to kill every single enemy present in the game in one run... And guess what, there is no information about how many enemies there is per level... What a shame!
So, here's the number of enemies per level to help you check your progression. Some levels needs you to take some particualr objects to activate some enemies, so i would recommand to also look for every items if your missing some kills.
Peru Levels
Level 1 - Caves: 14 kills
Level 2 - City of Vilcabamba: 29 kills
Level 3 - Lost Valley: 13 kills
Level 4 - Tomb of Qualopec: 8 kills
Greece Levels
Level 5 - St. Francis’ Folly: 23 kills
Level 6 - The Colosseum: 26 kills
Level 7 - Palace Midas: 43 kills
Level 8 - The Cistern: 34 kills
Level 9 - Tomb of Tihocan: 17 kills
Egypt Levels
Level 10 - City of Khamoon: 15 kills
Level 11 - Obelisk of Khamoon: 16 kills
Level 12 - Sanctuary of the Scion: 15 kills
Atlantis Levels
Level 13 - Natla's Mines: 3 kills
Level 14 - Atlantis: 32 kills
Level 15 - The Great Pyramid: 6 kills
There is a total of 294 enemies to kill in the game.
The achievement will pop at the end of the last level.
It's the first time that i wrote a solution, i hope it will help you to unlock this achievement. Good luck!