When attempting this achievement, there are an insufficient number of Risen Zombies within the first area of Stratholme where the 10 waves are fought. However, more can be found in the mini gauntlet after Chrono-Lord Epoch.
Therefore in order to get this, when you fight Salramm the Fleshcrafter drag and kill him in the Town Hall where Arthas encounters the infinite dragonflight. After you have killed him, make absolutely sure you do not kill any zombies afterwards, as they will not respawn.
From there, continue the instance as normal. Do not be concerned about talking to Arthas; just be patient and he will eventually run down in front of the Town Hall and will ignore all the zombies, and can be talked to safely from the steps without pulling any of them.
After you defeat Chrono-Lord Epoch speak to Arthas and he will open the bookcase, then have your tank run back and pick up every single zombie and kite them down the tunnel to where the gauntlet is. When the zombies get close, pick up 10-20 zombies from the gauntlet and AoE everything down, quickly picking up and killing any additional zombies required if you still don't have enough.
There are exactly 85 zombies in the first half of the instance. An issue with having the tank pull all the zombies is that there may be an overwhelming amount that would instantaneously stack up on damage and one-shot the tank. (assuming each zombie does 200 damage on plate, 80+ would be stacking 16000 damage.) Likewise, if the healer pulls aggro by doing direct heals then they will undoubtedly pull aggro and get one-shot as well. The tank cannot pull these zombies off without using a taunt cooldown or doing damage and accidentally starting the one minute timer. (Killing one zombie will start the timer, and killing another zombie will not start it again until those 60 seconds are up.) Paladins can use to completely absorb all hits under 500. A priest's will trick the mechanics to bring the healing aggro upon the tank itself and make it harder to shake these zombies off. If a healer can register themselves on the aggro table of all the zombies and run to the other side of the instance while spamhealing themselves, then they would grab aggro from the tank and bring them close to the start of the gauntlet where AoE casters can leave a trap down for all the zombies to just walk into. Warlocks can , mages can , hunters can , rogues can , paladins can , Death Knights can , etc. When killing all those zombies, the group will have ample time to pull the elite mobs and finish off the rest of the zombies necessary to get the achievement.
Due to patch 4.01 it is no longer possible to kite Salramm to the town hall and have the zombies spawn in, the new method NOW requires you to hold ground from where he spawns and let the zombies spawn and converge on you, when you feel there is enough zombies, you can use any AOE effect to get this achivement
Note- 4.0.3 This Strategy still works However instead of Pulling him into the town hall pull just outside keep in mind tank will have to keep taunting so he doesn't reset. once outside of the town hall AOE the Ghouls and wait for them to start re-spawning. Once they start finish him off then run inside. Defeat third boss. "Warning!-unconfirmed* Don't talk to Arthus untill you have gone back and Collected the Ghouls as talking to him might Force despawn them unless you have agro. My group had them all agro and had the last person talk to arthus at that time."
If that doesn't work just run the gauntlet without Arthus and collect all the zombies and then AOE them down, there is more than enough for the achievement