To get this achievement you must have the Hotline Miami dlc to be able to use the Baseball Bat.
To get this achievement, start the Hotline Miami job on normal, ensuring you have the standard Baseball Bat equipped as your melee. The proceed to kill as many Thugs using the Baseball Bat. When you run out of thugs, restart the mission. The kills are stacked against multiple attempts and will unlock mid-game when you kill the 30th thug.
Killing cops does not count towards this achievement, only killing a thug counts.
I found this easier to do using a Enforcer build, along with the Martial Arts and Pumping Iron perks from the fugitive class. Alternatively a build that is good for speed would also do the trick. As this doesn't need to be done in a single day the achievement is pretty easy to get.