This is a good question in which you can indirectly demonstrate your strengths and talk about areas of potential improvement. Be honest and positive.

"I usually find it easy to recommend students schools based on their preferences, grades, and interests after I've sat down to meet with them extensively. What's difficult is when I have to decide that their dream school is too much of a reach school for them to apply to given the limited number of applications available. Students and parents can be really insistent sometimes and they do get upset if they have their eyes set on a school that I need to tell the may not be the most realistic option."