What's the most challenging piece you've ever learned and how did you tackle it?

Probably Ondine from Gaspard de la Nuit.  I once had a skilled but sometimes unkind piano teacher who seemed to think that some music existed in a walled-off realm where amateurs shouldn't venture.  She was intrigued (and seemed annoyed) when I could play it through and said, "I'm surprised that it's less difficult than I'd imagined!" (She herself had never learned it.  

Stopped there, though.  Le  Gibet may have been feasible, but Scarbo remains one of the ultimate zones of terror.

Absolute rank untrained amateur here . :-) . A scientist type that strayed into the art pf classical piano playing. I heard an Ashkenazy recording on YT of Mozart K 414 about 2 years ago and not knowing any better decided to try learning to play it. (lol!) Had just gotten a piano at home. So here is what I have learn't on this journey..1. Got the score from IMSLP and started matching tempo with the Ashkenazy recording, then the Perrahia ( which is what I am still using) . 2. I have practised small sections ( sometimes just 2 lines ) for about 45 mins almost everyday. 3. Started with the second section ( slow ) which is in D maj. Found the rhythm extremely challenging.. after about 15 days it began to sound a bit like the recording which was a huge relief. lol!. Kept at it in slow tempo - started at 40 bpm so I actually hear how the 32 notes in bar 25 are to be played. Syncing with the left hand was monstrously difficult for me 4. After playing the second section for many days on end..wanted to see what the  1st section in A maj. This was way way too fast for me . I think I matched Maestro's tempo to be at 130bpm. So I slowed it down to about 60 ( Thank God .. YT has a slider on the settings for the tempo .. :-) Lol!) There is a lot of sound distortion but one can hear the articulation of the notes much better for learning.  5. Now after about 2 years - I have 25 pages of the score. No page looks terrifyingly difficult anymore.  Also I am able to join the recording with my score from almost anywhere on the 25 pages.  The biggest problems I am facing .. 1. When I listen to my recording of my playing on the iphone - I can hear all the mistakes, the halts and non musicality of playing.  People around me - very few have heard me.. say it it not bad .. but I think they are just being polite. I hear all the mistakes! lol!..Maybe largely because I have heard Ashkenazy and Perrahia play it amazingly beautifully and musically to perfection !.. :-) . I also like  Eschenbrenner playing this same concerto especially the second section.  Quite happy with this journey - Along the way I started watching the great masters on ToneBase - to fill the gaps in my rather non existent training. Heard the class from Garrick Ohlsen which led me to Josef Lhevine's book -got it and read it in full - would recommend it to anyone here - Then learnt about the Russian school of piano playing - from Anton Rubinstein to Lhevinne, Hoffman, Conus, Gilels, Richter. ( all from tracking the reference - Madam Lhevinne- in Sir Garrick's class ) . Also got to Jeffrey Biegel's amazing 20 min instruction on scale playing!. Wow incredible.. Brueckner's class and quite a few.. So the journey continues - most likely for another 2 years at least before the concerto may sound a bit musical. Lol!

The most challenging piece for me was the Polonaise-Fantaisie by Chopin. My piano teacher back in France came up with this piece, and of course, I completely fell in love with it. My favorite recording of the piece is by Kate Liu. She played it at the Chopin competition, but also at other concerts (by the way so interesting to see how differently she played it). The Polonaise-Fantasie is not only technically challenging for me, but also difficult from an interpretation’s point of view. And it was also the longest piece that I had learned so far. You basically can’t rest your mind for 13 to 14 minutes. I’ve learned the piece section by section, then slowly putting several sections together. I took my time to let the piece develop. Voicing and phrasing were challenging for me. But the most challenging is in my opinion to keep the piece together. I invented my own personal story that I try to tell when I am playing it. That also helps me not to “panic” when I  reach the technically challenging passages. I’ve also realized how important it is not to let yourself being carried away emotionally when playing, but still being in control. I played the Polonaise-Fantasie for my LTCL exam at Trinity College last summer. As an amateur pianist that was one of my goals: to pass such an exam. Now, I have put the Polonaise-Fantasie away for more than a year. I actually want to get it out again, when this year’s Chopin competition starts. Again, I will start practicing section by section and give it the time it needs.