Chapter 1- Mission 3
In order to defeat the iron Raveni you must target the padlocks on its armour, some of the locks require multiple strikes, they are much smaller targets.
the method i used was to destroy the padlocks on both legs, then chop off a limb, climb up his back, then jump in front of his face and while mid air use your RUne strike to target the padlock on his helmet, this must be done twice to remove the helmet. once the helmet is off then lop off his head and the achievement will unlock