This comes straight from the steam walkthrough wich can be found here.https://steamcommunity.com/sharedfiles/filedetails/?id=19665...
Credits to Omega Ridley
Defeat every snake in Decay.
Notes: There are seven snakes in total in Chapter 7, Decay. The best strategy is to shoot them from afar, as they’ll flee and then burrow into the ground if you approach them. If they manage to get away, you’ll have to restart the level as the achievement will become impossible to complete.
The first two are near the start of the level. As you enter the large swamp area, you’ll be ambushed by a group of demons. Defeat them and head east, drop into the water, and look for a chest to the southeast. The first snake is next to it. The second one is found about twenty meters north of the first, under the wooden bridge.
The third and fourth ones are located near the entrance to the tomb of the warrior. As you make your way up the steps past the horse-blocking seal, look west down the hallway carefully, and you’ll spot the two snakes running away. Use Strife to take them out before they vanish.
The last three are found as a group behind a secret waterfall to the southwest of the steps leading to the tomb of the warrior. You’ll have to grapple across a chasm to reach the platform, where you’ll get attacked by a couple of demons. To the left are some stalagmites, and behind them, almost invisible, is the hidden entrance to the cave where the last three snakes are found. Take them out to unlock the achievement !
And in case all of this wasn’t enough, I’ve prepared a map for you to use as a reference: