I got Find '250 epic cards.'
No matter how man chests I opened, I got none. I even opened the event chest (I got some duplicates for a legendary card I already had, which didn't help).
The only way to get through it was to fight the third boss over and over again, but since his respawn is like an hour, and you only get 15 cards at a time, I would have had to do like 17 fights spaced an hour apart with only three days left. It could have been done, but it would have been too much effort because I would have had to fight log in six times a day for the first two days, then five times for the last day, and still I would have had four quests left to open the fourth level.