Sworn enemy.

And if Anwen is used as a hero, such things will happen a little more often...

Her ability increases the damage against a sworn enemy by 2% per level. That is, at level 20, the damage against sworn enemies will be 100% + 2% * 20 = 140%. If there is also a successful attack against a sworn enemy... then instead of 300% for other heroes, Anwen will have 420% (without taking into account the Elven Luck ability).
That is, where a squad of Ossirian dancers will defeat 100 zombies (300 with a successful attack against a sworn enemy), a squad of Anwen's dancers will defeat 140 (420 with a successful attack against a sworn enemy).