Turn this challenge into a positive! Sometimes we view challenging tasks as things we are not good at or an area where we lack training. If you think of a particular piece of equipment that you find challenging to repair because you haven't had a lot of experience with it, go ahead and let the interviewer know. Perhaps it was a challenging repair because it took you longer than expected to fix. Maybe you had to rely on a manual to aid you in a fix. Admitting that something was a challenge because you are unfamiliar with it is not a bad thing. The interviewer would rather hear that you face a challenge head-on versus seeing you come across as a know-it-all.
"The most challenging equipment I have worked on recently was a new model mammogram machine. It was challenging because the software system was one of which I was not familiar. I love a good challenge, though! I went to the manufacturer's website to find a downloadable manual, dove in, and got the job done in under 3 hours."
"While attending university, I found most of the coursework challenging; yet comfortable. I did have to work more diligently, and even hire a tutor when it came to the module on radiographic and fluoroscopic x-ray equipment. I ended up completing this module with a grade of 86%, so my diligence paid off. I do look forward to further exposure to this type of equipment."
"Earlier in my career, I found oxygen systems the most intimidating. At this point in my career, I am comfortable tackling any form of equipment. In addition to my years of experience, there are so many options available for quick answers such as online manuals."