Killing bots seems to fulfill the Death Seeker challenge, which completely nullifies any challenge that it may hold. I don't think this is intended. I found this out by playing on the SG x1000 MvM server, and it works on machine attacks bots. https://imgur.com/a/SWkrzxc
Resolved!
Not Fully resolved - I still get progress by killing robots, as tested on DE x10, SG x1000, and DE x1000. https://imgur.com/a/obbkmp1
Fixed!