There might be a corner case where you've hit OP while killing the last set of trash mobs before a boss and you run into the boss with the option to either reset your rotation with HS->Maim->SP (to refresh the buff) or just finish with Mythril Tempest, refresh the buff, get 20 beast gauge. Since the latter is 1 move, gets you 30s on that buff and 20 beast gauge, I'm guessing that is the only corner case where you'd hit an AOE move for a single target.