I remember on my first play through, I kept losing to Seymour on Mt. Gagazet, Yunalesca, and Evrae. The same goes for the next few play throughs I did after that.
But on the file I started earlier this week, I have dominated. I am at the Bevelle cloister of trials right now. And of all the bosses so far, I've not had to use more than 1 healing item for each boss. Except Evrae, I used a few al bhed potions.