Many people seem to think that the achievement can only be done in the straightaway before Malganis after the third boss. The problem with this is that there are many many hard hitting elites, and right now people simply don't have the gear to tank / heal through blitzing through all the mobs.
However, there's a much easier way to do it. After killing the third boss, go back to the start of Stratholme. You'll notice all the zombies in Kings Cross and the town respawn, there's approximately 60-75~ish Risen Zombies in the entire town, and they all walk at 60% normal run speed (meaning you can out run them, easily). So kite them all up and lead them to the straightaway where the other zombies are, it will take a few minutes since they walk so slow but they do not leash. After this you'll probably have to go 3-5 elites into the alley to get the full 100 you need for the achievment, you can either blitz this part or have a healer/tank that are smart handle it =).
TLDR:
-Collect zombies from town after you've cleared first 3 bosses
-bring them to back alley
-profit
**important**the one minute timer for the achievement starts ticking after you kill a single zombie, and doesn't reset until after 1 minute. Make sure not to have ret aura/thorns/etc to accidently kill zombies too early.