Atori-Tutori ??? is a jerkori. We were doing pretty well until the first tail wore off. The guy to use the tail next was a little slow to react and we lost our DD as a result, though even then we were still doing a decent job. We got him down to 6% (according to Linkshell member) and then... I guess he activated Invincible + Party Obliteration.
One second after using Invincible (as advised), my health went from 1665 to 0. I... could not comprehend.