I'm having some difficulty with the final two bosses in the Via Infinito (Paragon and Trema) in FFX-2, though admittedly they aren't so much a case of "why won't you die" as "why can't I have time to get a single hit in?", thanks to the miracle that is the Catnip accessory. They literally pull out insta-kill attacks right at the start of the battle, every time, and despite having everyone leveled to almost Level 80, with Trigger Happy Lv.3, it makes zero difference. Otherwise, pretty much every boss in the first .hack// game was like that, taking massive amounts of hits to kill. In fact, fighting the bosses for so long was the entire reason I gave up on that franchise, which was otherwise good apart from the aforementioned bosses...
And yes, I am telling this FFX-2 story here in the hopes that someone can give me a good way to win those two fights, by the way... (hint hint)