One of the hardest pieces I have ever played, and the hardest for the harmonics, is the Serenade by Parish-Alvars. The entire first page is chords of harmonics, with the left hand playing either 2 or 3 harmonic chords on every beat, while the right hand plays the melody entirely in harmonics. It nearly killed me to learn that page, and I could only practice it for a minute or two initially before my hands would start to tense up from the awkward position. When that happened, I’d just skip down to the bottom of the page and practice the rest of the piece(which is not harmonics). I figured that by the time I had learned the rest of the piece, I would have learned the first page and built up enough stamina to get through it, which is what happened.
At first it was very frustrating trying to nail the harmonics, particularly the 2 and 3 note chords, all harmonics, in the left hand. I had to hold my hand in a very particular position and place the heel and side of my hand just so against the strings to get all 3 harmonics to sound. But I eventually got it, without ever marking the strings. I think it just takes practice, but eventually you learn the position for the harmonics in any given piece. Also, some harps produce better harmonics than others. Oddly enough, i think that nylon strings, even in the 4th and 5th octaves, produce better harmonics than gut strings.