I think the timing is key here because I get one every fight I get into litterally every fight my conjurer swordsman gets into especially when my summoned flame atronach keeps them busy while I go for a back stab
^ This.
Timing is indeed the key to getting a killing blow.
I also get it in every single fight on my warrior/archer.
I think the key for a direct melee fighter such as the warrior is to time the counterattacks, i.e, when the enemy is about to strike, you hit just before your oponents is midways in his/her attack. (Might be wrong but at least that's my only logical explanation for that setup)