I found the best place to get this was in the main game, playing level 4 in chapter 3, called "The Keep".
Near the start, you will go up the hill, and to the right, and come across a keep (on the left), with loads of zombies in it (they can't get to you as the gate is shut). In there are a zombie mix, but also the armoured ones. Shooting can be tricky, as you need to crouch/back up until you can snipe through a hole, but from here you can take your time, and try and aim just under his faceplate.
That said, I maybe "lucked it" here, as what happened is one of them turned his back on me, and I shot him in the head. He died and whilst his faceplate flew off in death, it still counted.
If you find this bit too tricky, keep going a short bit, and after the summoners are killed in the windows, mop up all stragglers, and approach the dead end (where the explosives are). 3-4 armoured guys will come out of crates, but they are slow, and the section behind you is nice and straight, so you can back up, and take your time. Good luck!