I've found that using the Achievement app's new Snap feature can help out here. When you're getting close to finishing, simply Snap the app and then just kill Zombies. If you need that particular class of Zombie, then the progress bar will increase, and if you don't need that class of Zombie then it will obviously stay the same.
This will then let you know which type of Zombie(s) you need to focus on to finish the achievement.
I've noticed this can glitch out sometimes, if the bar doesn't increase then quit the Achievement app and then relaunch it, and it should fix itself.
Edit: Sighris pointed out in the comments that every four kills of the same enemy type would make the bar go up 1%, so make sure you're counting it then instead of every time you get a kill.