I show you how to get the achievement in campaign mode. The first snatcher appears near the end of act 3-6. In case you miss it here you find another one at the beginning of act 4-1. To unlock the achievement let the snatcher eat a partner and then shoot at his belly to rescue him.
You can also get this achievement with Horde 3.0 mode if a snatcher the boss from a 10er wave (every 10th wave is a boss)