The fight I feel is the most unfuckingfair is Touhou 8 Stage 4. Yes, you heard me right. I think both Marisa AND Reimu are pains in the ass to fight. On normal, I can barely dodge fucking ANY of their damn cards. They're just BARELY worthy being their own final boss fight, let alone a Stage 4 boss!
AND IT'S NOT BECAUSE THEY FUCKING SHIFT DEPENDING ON WHATEVER. I'VE PLAYED LLS. I CAN BEAT BOTH OF THEIR ASSES FINE IN LLS.
If you want a fight that's more recent, try every Hard Mode fight on UFO. Nazrin in particular because Pendulums and Kogasa because rain spellcard (Yes, that's my only problem with her), Ichirin because fists and dense bullet formations. Haven't seen anything past stage 3 because Ichirin kills me every damn time.