Rainbow Six Extraction sludge monsters are causing more trouble than you might expect from such innocuous-seeming blobs.
Completing the Caustic Buddy challenge is hard enough on its own, but throw in bugs as well, and you’ve got a frustrating situation. There is a fairly simple workaround to deal with that bug, though it still requires a bit of luck.
Rainbow Six Extraction sludge challenge - How to kill a sludge without dividing it
The Caustic Buddy challenge asks you to kill a sludge without dividing it, though if you’ve busted any of these viscous monstrosities before, you know most weapons tend to split them on impact. Your best bet for destroying a sludge in one hit is using a powerful explosive weapon. The Fragmentation Grenade is a good choice, as are the Claymore and Arc Mine.
If you have an Operator who uses a shotgun, though, you could just walk up to the slime and end its day with a single blast. You’ll need to get right up to the slime before shooting, though, or the shotgun may still fragment the slime.
Rainbow Six Extraction sludge bug in Caustic Buddy
The trouble is actually finding a slime. At the time of writing, there’s a bug affecting sludge spawn in Rainbow Six Extraction – but only if you’ve started the Caustic Buddy challenge. Sludge stops spawning for many people once the challenge begins, though some have found a way around it.
If you can’t get sludge to spawn, try heading to the San Francisco region. Players have reported good luck on the Enterprise and Tenderloin maps, though it's unclear if this is just luck or something to do with the bug. Set the difficulty to Moderate or higher, so the Spontaneous Sludge modifier gives you a greater chance of encountering the creature. Spontaneous Sludge has a slight chance of making a sludge spawn after any other Archaean is defeated.
It means taking on a lot of enemies for a chance at a sludge just to finish one measly Rainbow Six Extraction challenge, but the experience is worth it. You'll need to heal accordingly if you want to stay alive. The Buddy Pass system isn't up just yet, but if you've got friends you can team up with, it'll make the process much easier.