"Known spawn location (second floor):" is misleading since this zone has a "ground level" entrance and then two basement levels. This mob spawns on the first basement level. There is no second floor, as the term is generally understood.
Casts an annoying irresistible 1-minute mez. If you're using a tank merc, it better be able to last a minute without a heal. I don't know what happens if you totally solo this mob since he's supposed to mez a random group member who's NOT at the top of his hate list. I just brought in a box alt and that eliminated the problem since the mez drops if you kill the related add.