A great spot to do this is in the HL2 Chapter 10 AntiCitizen One. When you enter the tunnel there'll be a few opportunities to hit enemies with the man hacks, but when you get to the end of the tunnel and talk to the guy who says he'll blast the dead end open for you, some man hacks will come after you as well as about 5 combine soldiers. Just drag the man hack in with your grav gun and hit the soldier with it. This spot is great for people who have a crap aim like I do when shooting the man hacks ^_^
Also note you don't have to get them all in one play through, oyu can start a new game and any that you've hit in the past will still count - go to the achievements menu in the game to see how many you need.