I find this Achievement easy. All you need to do is start a Skirmish with you being the Combine and the enemy(Easy) Rebels. As the Combine, meet the requirements to build a Headcrab Canister Launcher(HCL), Which is to build an armory. Now, build two or three of these HCL and wait for your enemy to attack one of your control points. When they do, double click your HCL to select all of them. Deploy Fast headcrabs on the area, Which need to be visible, and watch as the headcrabs turn your Enemies into Zombies. Keep doing this and you will earn this Achievement.
Have fun with your new Achievement!
