In Lab 6 there are two tool boxes underneath the reverter. The Alien is under there (it's a face hugger). You get the achievement by walking up to it though. So you'll need to do a custom level and make sure your death height is the smallest setting because you need to be under 10 cm to fit on the toolboxes.
To do this quickly just set your death height to the smallest, your starting height to the smallest, and set your timer to like 2 or 3 minutes. Just stand infront of the tool boxes and the second you are small enough just run up to it and give it a hug.