I don't think 'how many times it kills me' is a fair measure. There are plenty of bosses that kill me loads until I work out how to avoid/counter/negate each of their attacks, but that can take time to work out, especially when they have multiple phases. In 2D games with quick restarts I've often lost count of how many attempts I've made (Cyber Shadow's final boss was 30+), but if it's possible to get through while taking zero or minimal damage with a bit of pattern recognition and perseverance, and where every death feels like it's from your own mistakes and that I'm learning the fight and making progress, I don't really consider it hard.
The ones I think are hard tend to be those with attacks you can't dodge, with hit boxes that don't seem to make sense, where survival feels more luck than judgment, or where you're reliant on the AI not choosing a set pattern of moves in a row that just end you. The final boss of the original DS version of SMT Strange Journey comes to mind, where it's game over if the MC dies but the boss *might* just choose to waste you in the opening turns.