It took me a while to discover that Karl was evidently the cause of this bug. What kept happening was this:
When playing, I kept getting enemies that were drifting across the screen that would not die nor take any damage, so far as I could tell, until a crusader such as Karl or Veronica shot them. This kept happening regardless of my dps, are, or objective. It first happened during this last week's challenge, and again both during the Broot and Robbie Raccoon recruitment missions. Refreshing the game tended to clear the issue for a while, but it kept coming back. Clicking on the enemies would do some damage, but it took an autoclicker to hit them fast enough to kill before a crusader could shoot them.
I just removed Karl from my formation, and suddenly the issue was fixed. I had used Karl earlier because of the challenge and hadn't bothered organizing my formation properly beyond making sure my speed boosters were in play, as the level I was dealing with didn't warrant it. As such, Karl was landing in my formation by default. I do not know how, why, or for how long he has been bugged, as I very rarely use him, but he seems to be the heart of the matter. Should the bug return even without him, I'll update this post.

Maybe it's his "Split the Evens?" 200% DPS hit on one monster would kill them in that case, along with clicking, since it uses all of your total damage.

I don't like Karl because of that.