So this is a level 5 only achievement.
Part way through the level, you will get to a clearing with two knights. Let them spawn a watcher from their back and then start taking shots at the glowing parts on the knights shoulder armour. Once they pop off, back away and let the watcher repair the knights. You'll know this is done as they shine an orange light and then there is a little spark animation. Now, kill that knight and the achievement is yours.
A video guide can be found here: