Help on skill first assault

Current upkeeper: Nobrain
Using time      : 6
Skill cost      : 12
Skill type      : offensive (single)
Affecting stats : Str, dex
Affecting skills: Anatomy, focus power, stun, weapon skill, assault, strike, this skill
This skill allows  you to sneak  behind your opponent  and hit it  hard with a
deadly and stunning blow. You can't use  this skill if you or the opponent  is
already fighting.

First aid < List > Flanking