In this video guide I will show you how to defeat every snake in Decay. So to start off there is 7 snakes in total. They are located in three different areas. An important note to take is that if you fail to kill them quickly, they will leave the map. Then you will need to restart the mission over.
First area is right next to where you start, also right by the trickster door.
Just head to the right side of the map into the swamp area. When you go into the swamp area there will be one on the bottom part of the loop, and one on the top. If you are riding on the horse, make sure to use Strife. While riding as Strife you can aim his guns to see the snakes when you get close.The second spot is further up north by some stairs. There will be a conversation at the bottom of the stairs for further reference.
Head to the top and continue straight down that hallway. A little over halfway there will be two snakes side by side in the hallway.For the third spot. You will need to head back downstairs and continue on with the map.
There is only one way to go really. So follow the map for a bit. You will eventually notice a boss room. Go past it and head down south a bit. You should see a waterfall on the left side. Walk through it and find the last three snakes.
Once all 7 are dead the achievement will pop.