If multiple warriors with this quest active are in the alliance, the NMs only have to be fought once.
If you (while solo) disconnect during the fight, call for help, or your party/alliance loses claim on all the NMs, you will not be able to get the CS. Even if you zone back in quickly and kill the NMs, you will not get it.
If party holds hate you can still complete the mission as long as you get hate on the NM before last one is killed, then touch the ??? as normal.