You should say:
-What was the challenge
-when and where did you complete/met it
-why was it difficult
-how you felt about it?
Sample Answer to Describe a difficult challenge that you completed/met.
Life is full of challenges which sometimes bring forth amazing experiences afterward. I too had encountered such sort of situation in which I had to struggle a lot and this happened when I was pursuing my diploma in fashion designing. I remember that I was given a group project related to Halloween eve to submit within 2 days. As I was never having experience of teamwork, it was an arduous task for me to work on that project in a team.
At first, I faced some inevitable arguments with other members of my group regarding the presentation and ppts, though after some time I started realizing that to make this project a success, it is a prerequisite to be a great listener. So, once I began listening to the ideas and understanding the perspective of my teammates, I managed to get an incredible score in that particular subject of fashion designing. It was an extremely daunting and mentally exhausting task for me, however, the right decision at the right time aids me in meeting the deadlines and attaining good grades.