Ahaha Rust Tyrano!! Ever heard of Ruby Rings? Sold in 65mil BC? Yeah, put 3 of those on and Rust Tyrano just takes a little while, but otherwise you'll kick his ass. There's a difference between getting your ass kicked when you know what you're doing, and getting your ass kicked when you don't know what you're doing.
I haven't really played much of the old games, just about everything in Bof1 was mega hard, I hated how I got the bosses down to the little red life bar, and I had to do like 5x the damage to bring it down there to kill it.
I guess a good mention would be Yunalesca, she was pretty hard to me. Hardest boss in FFX ( I didn't find the final aeon very hard since I got a lot of opportunity to easily prepare for it. )