Mid-point boss requirements are a punishment to players who join in-progress runs. This has happened a few times in the past month now. The optimal way to work around this nuisance at the moment is to requeue for a normal dungeon after, complete the requirements, and to abandon the group, which furthers the problem. How this ever got implemented into the game is beyond me. It's such blatantly anti-player and anti-teamwork design.
-Only require killing the final boss in the dungeon, always
-Allow us to not queue with in-progress groups