Man, Sephiroth in 'Final Fantasy VII Remake' is no joke—he’s like the ultimate test of everything you’ve learned up to that point. First off, make sure your party is leveled up decently; I’d recommend at least level 50 for Cloud, Tifa, and Aerith (or Barrett if you prefer). Gear matters too: equip the 'Gotterdammerung' accessory if you’ve got it (from completing all combat simulators), because that instant limit break at the start is a lifesaver. For materia, stack healing and revival stuff—'Pray' and 'Chakra' are great for sustain, and 'Revival' is mandatory unless you enjoy watching Game Over screens.
Now, the fight itself is split into phases, and Sephiroth loves to spam unblockable attacks. In phase one, focus on dodging his 'Shadow Flare' and countering with Tifa’s combos to pressure him. Once he’s pressured, switch to Cloud and wail on him to stagger. Phase two gets nastier with his 'Heartless Angel' move, which drains your HP to 1—have Aerith or Barrett ready with 'Magnify' + 'Cure' to heal everyone instantly. Save your limit breaks for phase three when he starts flying and spamming 'Supernova.' Honestly, it’s a marathon, not a sprint—patience and well-timed counters are key. And hey, if you fail, just remember: even Cloud got wrecked by this guy the first time around in the original game.