On keyboard, press "T" to tag special enemies.
A decent place to farm these special Zombies, I found is Moscow Episode 1, as long as you don't "rush" to the next objective, the Zombies should continue to Spawn. I believe I may have tagged atleast 5-10 special enemies per run.
You can also work on other achievements whilst here, such as upgrading certain weapons, using breach charges (take fixer class) and set up defences (the end where you have to protect the helicopter). There is about 3-5 defences that can be used so is a nice bit of progress to towards that achievement.