This achievement can be gotten in a matter of half an hour. All you need is access to the south part of the first map. First you will need to play at night
as Ghûls only show up at night. If you aren't playing at night, select a tower and you can Advance Time
. This will change the time of day.
Now travel to Udûn Crossing
. There are a few spots where Ghûls can spawn. First, look to your south-east like I did in the video. If you see Ghûls or nothing, go there and see if some Ghûls spawn. If they do, just keep killing them. This is a great spot as it provides 12 to 20 Ghûls every 5 to 10 seconds after you defeated a wave.
If the spot is taken, advance to the south and enter the cave like I did in the video. The cave can be filled with Orcs or Uruk-hai in which case you need to fast travel to the tower to reset the location. Repeat this until you get either Ghûl spawn to work.
In the first part of the video I advance into the caves with and stop at the fire with a slave. I stopped the recording to soon as Ghûls were spawning readily right there. If you see nothing or just a slave, just stand there for a few seconds before you decide to reset the location by fast travelling.
Once you got Ghûls to spawn, proceed with the killing until you see some of them start to flee. This will be indicated by a red symbol over their head. Do not kill those and if you use finishing moves, always take care to leave a few Ghûls standing. The survivors will attract more Ghûls resulting in new waves spawning.
The achievement shouldn't take you more than half an hour to get using this method.