Pretty easy, just a little time consuming.
Do co-op, the "practice" mission. At the point where you run into your first rock launcher (demomen), and 2-3 other guys you will have to activate a sliding door. Once you get to this point, sprint past all the enemies, don't kill them let them slow your team down.
You will get a audio telling you about shield guys and how they knock you down, at about this time you turn to the right down a path and run right into a shield guy. Let him knock you down, focus your fire on his friend (headshot) and if you're lucky you can get a headshot on the shield guy as well.
I was getting 1-2 like this a match, then finish match, quit out and join another.