There's another thread going on here about final bosses that are pushovers (whether they're just easy, or just a final boss for plot reasons, like in FFX after Braska's Final Aeon). That got me thinking on final bosses, which got me thinking about how I've gotten dominated by plenty of bosses in my day. Sometimes just me being a noob, sometimes they're just really friggin' hard. I got a few.

Before that, I think it's worth mentioning sometimes RPG bosses are a bit on the easy side because I horde my good consumables for like the entire game. Sometimes easy because you overlevel. Sometimes hard because you underlevel. This thread is probably better suited for not RPGs, but I have way more experience with RPGs.

The Radiance of Hollow Knight. I died on this dude (?) so many times, finally looked up charm suggestions online. Made it... easier.

The Profound Darkness of Phantasy Star IV. First encountered this when I was a kid, got owned. Your choice of 5th member makes a big difference, can make your life a lot easier. Ya know, I'm not quite sure this should be on the list, but I struggled with it.

Quite a few of the Final Fantasies have bosses I struggled with. Chaos of I, have to dish out enough damage before he heals. Kefka of VI, Sephiroth of VII, got a gauntlet first. BFA of X, that dude's epic if you run straight through with no grinding. Same for Bhunivelze of XIII-3. I don't recall the others (II, III, IV, V, VIII, IX, XII, XIV, XV) being challenging.

Fou-Lu of Breath of Fire IV. May be a case of underleveling, or poor party choice, but he owned me a couple times.