I personally thought this achievement was going to be a major pain, but it's actually a bit of a breeze, provided you have three things.
#1- Good guns.
#2- A good team.
#3- A good defensive position.
If you have all three, you can easily get 30+ headshots a round. When I got this achievement, I started actively going for it at level 17. I began that level with 40 headshots, and had the achievement by the end of level 22, getting 110 headshots in 5 rounds (the sixth round was dogs). And I'm by no means a headshot master- prior to that, my high mark was 80. Now, to go through those points one at a time in greater detail.
#1- Good guns doesn't mean the Ray Gun, the Shotguns, or the Wonder Waffle. Those are great guns for regular zombies, but terrible for headshots. The best headshot guns are the Browning and the MG42 because they're all business- they kill a zombie quickly and let you move on to the next one. Remember, in order to get 30 headshots a round, you're going to need to get over 30 kills a round. I'm not usually a doubletap fan, but it'll speed up your killing and let you get even more headshots a round- just remember to burst fire. Also, Speed Cola is a MUST, because MGs take forever to reload, and Juggernog is useful to prevent you from losing those two perks. If you can't get an MG42 and a Browning, the PPSh also works, or if you're really in a pinch you can buy the Thompson off of the wall downstairs.
#2- Your team should be well-practiced at working together, and should be able to consistently get to levels 20-24. It really helps to play regularly with the same crew so you can get familiar. You especially need some guys who aren't glory-hogs, and who are willing to hold a quiet window and watch your back while you go for the achievement. I've had some good luck with 1 random player, but if you're playing with 2 or more random players, you're probably screwed. A good idea is, if you ever play with a random player who is a good team player, send him a friend request until you have some regulars you play with frequently. It's more important to have great team players than great players.
#3- your position is very important. Some people like holding windows because you have lots of time to line up your headshots. I don't, because traffic is so light you won't get more than 10 or so a round. 10 a round is decent... but 30 a round is much better, don't you think?
Basically, what you're looking for is a location with a long sight line and heavy zombie traffic. Ideally, it's a hallway or something that will funnel zombies so they come at you single-file. The absolute best location is the balcony on the 2nd floor of the main building, right next to the zip line. Have someone hold the window on the outside across from the zip line, and the other three players hang out in the hallway facing the stairs. You should stand in the middle of the hallway in front of your two buddies. Stand up (don't crouch), and aim your MG at around the waist of the zombies. When you burst fire, your MG should walk up to head-level (adjust up and down until your burst ends at head-level). Since you're standing in front, all of the zombies will come straight at you, which makes aiming a breeze. After that, it's just a matter of lining them up and knocking them down as they come at you, letting your buddies help wipe out any that get close to you while your fourth buddy holds the window. Unless you get a ton of Max Ammos, you'll run out eventually- at that point, let your two buddies in the hall hold it until a Max Ammo drops and get back to work. I don't think it's worth it to trade in your guns once they're empty- in my experience, you should rarely be waiting too long before a max ammo drops, and you'll regret giving up the better guns once one does.