Alter Aeon Online Help

Keywords are:  'warrior skill flanking attack'

Skill: flanking attack          Lvl 29 Warr  (21%)
                                (helpful)   Requires: retreat
                                Group: Warrior group tactics

Usage:
flank <ally>

This high level warrior skill is used in groups to outmanuever an opponent.
A fighter moves around to attack an enemy from a more advantageous position
while its attention is focused on another target, usually a tanking group
member. In addition to inflict heavy damage, this flanking attact can distract
the opponent long enough to create an opening for their target to get in quick,
powerful attack.

While this tactic can be effective from time to time, it cannot be used
often or the target will begin to anticipate when they are being flanked.