Might be colored by my experiences so far, but I'm about ready to put the Extra Stage as the hardest among the Windows games. The stage portion is mostly trivial stuff, but there are exceptions such as those random spam sections before and after Doremy that are easy to screw up, those bean-spamming big fairies that you have to shoot down fast or the last criss-crossing bean section, which is easily high-end Lunatic tier and rather long. Doremy's mostly easy peasy, but Crawling Bullet likes to be a jerk from time to time and has that double kill bullshit Nueball had going unless you bomb before the card as Reisen.
Hecatia's nonspells are unheard-of brutal by Extra standards (barring Mokou's rings of death) and have tons of HP - the pink one is obviously the worst due to barely enough space to squeeze into, but the blue ones also have the tendency to overlap just in the right way to box you in, and Junko's nonspells are basically soft resource caps unless you're very confident in your microtaps. The first and the second card are easy once you get the hang of them, but the gloves come off by Apollo Reflection Mirror, which demands some very quick thinking. Trapped Rat looks like a riddle and Rain Falling in Hell is very much manageable, but Hell's Non-Ideal Danmaku screws pretty bad with my reading ability. Lunatic Impact demands on-point microreads, good rhythm and the ability to keep calm under pressure. Pristine Danmaku might as well be another softcap. As for the survival card, depends on whether those things are static or not, but the later barrages of all three phases demand consistent guessing of the trajectories of tiny bullets on a split-second basis that I certainly cannot boast. Finally, the last card seems pretty manageable, but I haven't reached it yet.
All things considered, I do not want to prononunce LoLK Extra as the hardest Windows Extra yet - UFO certainly boasts a way harder stage portion and Mamizou's latter half of the fight is brutal enough that I feel reclutant to dethrone her from the hardest EX-boss on my list. But LoLK is easily up there among the hardest - a far cry from DDC, where I cruised through on the first day thanks to the more broken shot types (or the likes of PCB or MoF, which I could just resource through after getting the hang of some patterns for the matter). This time, I advance forward inch by inch purely by dropping man-hours into getting the hang of everything.