Ménage à trois Achievement
Ménage à trois25
Complete 5 quests with 3 co-op partners.
NOTE: This achievement can only be obtained with players with the NTSC-J version of the game, whether it be online or system link.
Despite the achievement's name implying only three people will be involved, this is actually going to be a Menage a Quatre. Get a full party (may as well get "Originality" at this point) and do any five quests. Obviously the same five you did for "Together in the light" are the easiest. In fact, you can couple that achievement with this if you want.