I managed to get my elven figher/mage/thief's AC to -16 just using equipment, but the achievement wouldn't show up. Did I miss something? Is there some other condition?
EDIT: Seems it only pops if the top character hits the AC -15 mark, since when he did that after I did some buffing, the achievement showed up. My FMT was 2nd in the lineup so maybe that was why it didn't then.