Bored of the challenge!
I appreciate that for the hard core gamers getting to 900+ stars is the highest priority of the challenges. For me, though, I do not have the time for the dedication required to achieve this goal. Instead I'm forced to sit through 54 missions before the challenge difficulty catches up with my survivor level. As a result I find the challenge boring and unchallenging. The solution is simple. Introduce a sliding bar that allows you to chose your starting difficulty when you first open the challenge. This will allow those who have the time to continue to get their 900+ stars and will also keep the more casual user engaged and interested.