It's really stupid, it happened to me. You have to make sure you complete all of the objectives. If you miss anything it kills everyone. Also time allotted for GM level is not enough. It take so much time trying to figure out where to go and wait on waves of mobs to spawn. I got to the end boss with no time, takes all of my lives and when I got overwhelmed and died I failed the -blam!- activity. The chests are only giving glimmer not sure what completion gives, cuz I wiped.