I killed Viktor by losing the light from the top in the 3rd floor while he reached the end of the walking stage, but nothing happened. Is it a bug?
It is bugged. It’s one of the few challenges left on IO’s list of known issues. Technically, some people have unlocked the challenge by dropping the rig at the exact right time (I think it’s the instant when Viktor first steps under the rig), but I’ve been waiting until they get it properly fixed.
You can complete this challenge indeed.
You just need to pick the right moment (save the game just before doing) when Viktor have only just stepped onto the stage
Interesting. I’d given up on this being possible. Can you just go straight to doing this or do you need to do it following completing all story elements relating to it?
In case it helps anyone else, here is a video showing the timing where I finally managed to get this challenge to unlock:
I can’t tell if it was necessary or not but I did go through the story elements leading up to this just in case I needed to. Interestingly the time this worked I dropped the rig just early enough that the story didn’t get marked as completed. Just possibly this could give the developers a clue to the cause of the difficulty here.
In the version of v3.40.0, updated in 2021.06.15, this bug is fixed. I just tried, and the challenge was accomplished right after the target was killed on stage.