I. Haven't. Beaten. It. Or II, III, IV, V, or VI. Admittedly, part of this stems from a lack of interest based on a nigh-unconscious prejudice against the graphics and gamplay style of the older games. I haven't beaten XII, XIII, or XV yet either although that's mostly because I get sidetracked trying to go completionist (same problem with Rogue Galaxy) and I just never bothered to pick up XIII for the same reason I never played XI and XIV: didn't have the console I needed.

Square games I've had a legitimate difficulty in completing have been FFT and Vagrant Story.