The Orange Box

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.

In the second chapter of episode one, a number of rays must be fired into the core to contain it. After enabling the first ray, several groups of man hacks are released and more then five soldiers enter the room in pairs of two. Use the grav gun to grab a man hack, walk up to a soldier and fire it in his face. Repeat this five times and voila.

Got it the first time when I tried and it takes only a few minutes to get to this point after starting the second chapter.

Very simple and can be gotten throughout the game, you do not need to do it all at one time or with the same manhack. When you hear their familiar buzzing just pull one in with the Grav Gun and when you see a guard shoot it at him. Do this 5 times and this simple achievement is yours.

You do NOT have to shoot the manhack at enemies. An alternative way to earn this acheivement is to grab one with your gravity gun and hold into it. Doing this and touching a mob will kill it in seconds. I managed to get this in 'AntiCitizen One' killing mostly headcrabs in the lower tunnels, as they jumped at me they instantly died. Any enemy will count (except for barnacles and other manhacks).