***Be aware that you must do this BEFORE finishing Tyler's bar fights***
To obtain this achievement, you need to win 3 fights against street muggers.
To start a mugger fight:
1) Have at least $150 cash in possession
2) Move around the map
3) Mugger fight will randomly occur
After the 3rd fight, you'll go up against Big Bob. Beat him to unlock the achievement.
(I recommend a strong dodge build character to make this a lot easier)