ZOS mentioned this was due to "leet DPS" in the patch notes and claimed it was fixed. It isn't. Until they actually get around to addressing it, try using only light attacks and stop having more than one person focus each one at a time. Sounds like a pain, I know, but our group must have reset 8 times before we finally got it to work and that was the only thing that worked out of every single thing we tried. On Windows PC, via Steam.