A good place to get this is at Gang Headquarters in New York (red icons with a tower). There seems to be a high concentration of Stalkers around these locations. You can also visit them after you have taken them over, and Stalkers still appear to spawn.
You want to walk around in the alley's around the Gang Headquarters. You will start to hear whispers, which indicates you are closing in on a Stalkers. Activate Eagle Vision by clicking
, and walk in the direction the compass points you. When you get close to a hay bale, well or other hiding place, the Stalker will jump out at you. Press
when the exclamation mark appears above their head, and then
to kill them. Keep walking around and another one will likely already be highlighted.
Do this 30 times and the achievement is yours.
This achievement can be easily be unlocked at the same time as the following. Occasionally, you will be attacked by a Stalker on the rooftops, which counts as an air surprise attack. You can counter smoke bombs by chasing the gang leader, causing him to drop several in an attempt to escape. For more information, click here
The video below highlights how to unlock these three achievements.