As for what's actually being presented, leaving the monster claimed to the group until everyone is dead is a good solution. If they take long enough to rage the mob then that solution is enacted forcefully on the players.
It's not a good solution because you can have people alive in the alliance but not able to fight. As long as someone has Atma of the Apocalypse, you can effectively keep an NM locked down forever (until someone invokes a GM to make you give it up for taking too long)