There is a fundamental problem with having a healer and a tank round them up. How would a tank hold aggro on something they can't attack? If your tank can't handle rounding them up alone then try another tank, if you are the tank make someone else tank and go DPS.
As for actual advice. Even a relatively bad paladin tank can round them up easily enough. If they get in trouble Divine Shield doesn't deaggro them if you are the only one in combat with them (i.e. the rest of your group on the other side of the town hall). Divine Shield is also a great way to get them to cluster up in order to make them easier to AoE when it actually is time to kill them. A paladin can also heal themselves if they get a little distance between them and the zombies.
You have to generally wait for the zombies right outside the town hall to respawn while fighting the second boss, because those zombies were the last you killed when pulling the boss there. They all respawn on the same timer. Since there are 85 in the starting half you will want almost, if not all, of them in order to do the achievement.
Remember the timer starts when the first zombie is killed. If you accidentally kill one early, you have to wait the full duration of the achievement timer to allow it to reset on the next kill.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.