While I agree the issue is annoying in general, it's not a problem in this particular fight. As other people said, go up the stairs and you should have no problems taking them down.
It was my understanding if you go up the stairs you get hit for 9999. I certainly saw it happen once during our last attempt.

I'm kinda suprised that this functionality doesn't bother more people. They can keep us from safely nuking things out of harm's way without making it so you have to get up in a monster's face if they're on a hill before you can cast on them.