I did not make this guide, I only made a few changes. The credit for this walkthrough goes to Gimli from seriouszone.com
Play on Tourist where there is a total of 728 enemies.
I broke down the level into areas to make it easier to check. Since there's backtracking, some areas have a number to mark whether it's your first or second visit. Next to each area name I wrote the number of enemies in it and the total number of enemies in the braces. For the last two areas, I split the numbers into waves, because there's lots of enemies there.
NOTE: You will need to know where the secrets are located in this level. Here is the URL for all the secret locations in the game http://www.gamefaqs.com/console/xbox360/file/961109/58369
(Thanks Rebmob_x360a) You do not need to pick up anything, you only need to walk over the item to trigger the enemies to spawn. Unlimited ammo can be used.
Generous yard (secret) - 5 (5)
First room (elevator secret) - 48 (53)
Small room with +1 healths - 4 (57)
Sphynx courtyard 1 - 6 (63)
Buttons yard 1 - 20 (83)
Lever room - 9 (92)
Buttons yard 2 - 17 (109)
Sphynx courtyard 2 - 4 (113)
4 buttons complex (including the side rooms) - 9 (122)
Sleeping arachnoid - 58 (180)
Heart trap room - 16 (196)
Praying animal yard 1 - 20 (216)
Cross pool 1 - (Croteam secret + that other secret ) - 112 (328) - Note: when killing the kamikaze in the left trap, be sure not to move much from the +25 health, otherwise not all of them will spawn.
Amon-Ra statue room - 3 (331)
Cross poll 2 - (secret by the obelisk) - 22 (353)
Praying animal yard 2 - 12 (365)
Secret Goodies room complex (Gnaars and +1 armor secret) - 6 (371)
Fight music arena - 208 frogs + 60 kamikaze + 5 werebulls = 273 (644) - Note: Do not let the werebulls break through the door,when you have enough werebulls the door will open.(Thanks Sashamorning)
Last arena - 32 kamikaze + 12 kleers + 25 harpies + 4 arachnoids + 11 bulls & Biomechs = 84 (728)
(Thanks MrGompers for those last important numbers)
The achievement will unlock when you walk through the last door to start the cutscene.