Shin Megami Tensei: Strange Journey had Mem Aleph, the final boss of the Law and Neutral routes, and...wow.
High stats all around, infinite MP...immensely powerful attacks of each element, 100% accurate insta-kill attacks of multiple elements, crippling status effects against the whole party...and her resistances constantly change.
And that's just her first form. Her second form has a new attacks that's a 100% accurate insta-kill...without an element, which means there's no equipment or ability that can block it and it HEALS her by the victim's full HP! And if your main character dies at any point in the fight, it's game over. That's right. Every turn, there's a non-negligible chance that you lose-outright. I hit the level cap at the end of a rather long game, but had to give up because the final boss was just that tough.
Oh, and as Panzy said...Boobeam Trap. Uuuuuuuugh.