I don't think this specific mission has given anyone trouble yet, but I can't get past it. It's the last one (bottom right corner) of the 2nd panel. And before anyone directs me to gbf-english.proboards.com/thread/240/granblue-fantasy-ultimate-guide, I've been there. I've clicked on the bottom left button during raids to request help, it works and the bosses die fast, but the mission doesn't get completed. Any help is appreciated.
Problem is solved. I don't know why but the left most option that pops up after you request for help supposedly means "everyone". It had a exclamation mark on it this time, and I was able to complete the challenge. Please consider this thread closed.