Knock out 200 people. Use either fists or weapons, just make sure they are knocked out. Press to punch them, to use a weapon. Firecrackers make an easy one-hit knockout. If you do not have this achievement by the time you get the spud gun, that also knocks out anyone in one hit.
**You can check how many knockouts you have:**
Start---> Stats---> Fight---> General Fighting---> People Knocked Out