Phaleg fight in pso2.
Although this game is an mmo, whenever this fight got released, it definitely ramped up the difficulty in the game, and this was basically a trial of "are you actually good at playing the game, or have your friends/allies been carrying you this whole time?" It's a solo fight too, so you really have to try your best to beat it alone. You have to basically dodge most of her shit or she's going to literally clap your cheeks, essentially making her as hard as a dark souls boss except shes constantly taunting you because she knows she's overpowered.
Some skills let you dodge while you use them, which is something this person was definitely taking advantage of. Also she has a shit ton of hp which just makes the whole fight even worse. Second phase and such she also gains resistances to the types of weapons you use, so if you beat her ass with a weapon on prev. phase, she's going to take less damage from it on the second. (This person in the example used the same weapon.) Each phase has about 3 million hp, so it's a true test of dps and how well you can survive.