Phenotrans Nuisance Achievement
You killed 20 Security Guards. What did they ever do to you?
These guys are all over the place and this achievement should come pretty quickly. If you want to get it done quickly kill the guards around the guard tower. There are usually four or five around there. Then go back to the safe house to reset them and do it all over again. The guard tower also has a lot of guns and grenades to stock up on if you want.