I was looking at some skins I wanted to unlock and one of them was the Soldier helmet with the Zenith skin. It says the requirements to unlock the pieces is to master the Spartan Spree commendation.
I took a look at my commendations and found Spartan Slayer and Killing Spree, but no Spartan Spree
I haven’t mastered either one yet, so I was wondering which I one I am supposed to master to unlock the skin? 
Take a guess if it isn’t spartan spree which is the other option? Killing spree of course! It’s just the UI misleading you. Yes you do have to get the Killings Spree mastery and Spartan Slayer Mastery.