The way I have found to do this is in Mutation < Last Man on Earth. Start The Parish < The Bridge. Leave the saferoom and start walking across the bridge. This may be a system of RNG, but generally since it is only special infected on this mode, not a lot spawns. If you need more than a text guide for this achievement, here is a video:
Good luck and credit to GRE3NARROWFILMS for his/her video.