Hmmm, for me there is not one most difficult level, there are some very difficult ones.
13 would have been impossible for me to solve without a lot of help, since I don't play chess at all.
24 was very difficult, because upon the clues many different directions were possible, and I would have never thought of what you wanted. Without your hint, I don't think we would have gotten it.
38 took us long - it would have been easier, but we've tried other things for a long time