Back 4 Blood

At first this one seemed a little hard to accomplish. Thankfully its not too bad when you understand what you need to do. You have to kill a ^normal Snitch. You can tell which ones they are because they have a nice red fleshy part (Weakspot) as their elongated neck from their shoulders to their head:


H O W E V E R
before we go further. If you have the corrupted cards on your mission:

The Dark
OR
Job 10:22's 'Finale'

Each time you kill a Snitch; in its death it will alert the horde. That's because this Snitch is mutated and just likes to screech like that ¯\_(ツ)_/¯
You will find these mutated Snitchs do not have a red fleshy weakspot neck. It instead has a grey one with red spots on some tentacles at its base like below:


There is no one mission to direct you to as a Snitchs spawn and Corruption Cards are randomised. I did find on the mission set up of

Act 2, 6-1:
Pipe Cleaners
Recruit Difficulty

That there was a better chance at finding the Corruption card that helps with finding normal Snitchs. This Event card is known as Tattlers

You should have an abundance of Snitchs to kill with this Corruption card. A direct hit grenade will one-shot kill it. You can purchase the grenade at the starting Vendor. If you prefer something different... A barrage of fire from any gun to its weakspot will also kill it as there is a second delay before it will screech.

I killed mine with a direct hit grenade and I have sniped one and knocked it down, it got back up and then sniped it again (in the weakspot for the sweet extra damage!) and it did not alert the horde in this time

Good luck

There's a really easy way to get this one. I can't suggest a mission as.theyre random spawns, but its worth nothing this will only work wkth STANDARD snitch mutations, not the ones you get from contamination cards. Snitches buffed from contamination cards will scream even on death, so there's a little bit of RNG involved.

The easy way is, keep a grenade handy, go through levels on recruit and when you see a snitch toss the grenade at it for a direct hit. It's a one shot kill.

I found that Act II was the best act for snitches, and worth nothing that the normal snitches have a big red lump for a neck, whereas the special snitches have a normal coloured next with little red spots.

This is a video going over the Snitches get Stitches achievement and telling you the best ways to go about it any why you may have not unlocked this already:

In this video guide I will show you how to kill a snitch without it alerting the horde. To do this easier you will need a grenade to kill the snitch. Later in the game there will be snitches with darker skin, almost black. These are the ones you want to throw the grenade at to kill instantly without alerting the horde. Once you kill it with the grenade the achievement will pop.