One easy way to earn this is by playing the "Four Swordsmen" mutation.
- You start out with a Katana.
- The game will only spawn special infected (no commons), and the spawn rate is quite high.
- AI teammates cannot shoot the smokers.
When you see a smoker, move to the front and try to make him target you. When he starts to shoot his tongue, there is a split second you need to swing your sword right before his tongue touches you.