I have now experienced the same glitch twice while playing the Takedown.
In the Wotan fight, co-op splitscreen, I shot Wotan’s upper half with the face-puncher.
This resulted in the upper half splitting into several pieces, the pieces falling to the ground, and then de-spawning. Damage dealt was definitely not fatal.
No credit given for completing the Takedown.
As a result, surviving player was left in an empty arena; other player stuck in spectator mode.