Hey guys, check this link out: (look at the servant section)
EDIT: the link didn't work for some reason. Just go to the jp wiki and click on "Android version · iOS version / skill" from 2016-11-27 from the left sidebar.
It seems that Aura Espada can proc by normal atk, by defense, and by evasion, too. It looks a bit similar to ench's Magic Blade, having both a physical and a magical part. The magical part is no attribute and can spell burst. Also, Aura Espada can be used with Hide Attack, can shell break, and is affected by other buffs.
So, counter servant will still make use of this skill, adding another counter to their arsenal.