Being able to trudge through the tough and challenging times is a really important skill to highlight. Tell the hiring manager that you are able to succeed and push through.
"The most challenging task that I have faced as a QC Inspector was the recent implementation of our new facility wide safety program. It was very detailed and required a lot of training hours and re-certifications. Also, there was a great deal of resistance from many of the employees. In the end, it was a very successful endeavour and I'm pleased with how it turned out."