First, redefine “difficult” to be “challenging” which is more positive. Then, identify an area everyone in your profession considers challenging and in which you excel. Describe the process you follow that enables you to get splendid results…and be specific about those results.
Example: “I think every sales manager finds it challenging to motivate the troops in a recession. But that’s probably the strongest test of a top sales manager. I feel this is one area where I excel.”
“When I see the first sign that sales may slip or that sales force motivation is flagging because of a downturn in the economy, here’s the plan I put into action immediately…” (followed by a description of each step in the process…and most importantly, the exceptional results you’ve achieved.).   _{Your Comment}