Most recently, Final Fantasy X's Yunalesca.
3 excruciatingly long fights due to status effect bullshit. 3rd form will insta-kill party if you try to prevent said status effects or were unlucky enough to heal at the wrong time. It just became a fight of attrition until my healing items ran out and I put the game down.
How did you not learn after a first insta-kill?
It pissed me off too, but when i saw the opening move on her third form, i'd make sure i didn't make that mistake again.