There are 7 snakes on the 7th mission "Decay". Check out the video below to see the location of the snakes. One important thing to note is that if you fight or shoot around the snakes they will begin to flee. If they escape before you kill them then you will need to restart the level.
You can start the level, speed run to the snakes and kill all 7 in 2-3 minutes.
Credit to Wise Guides for the video.