The above guides are great but there is an even easier way of getting this, and you don't even have to craft ANYTHING!!!!!
In the Tower, in the same room as the vendor, there is a guy named Buckshot who has a challenge called The Killing Guide....
The first challenge is to kill 40 infected with a baseball bat in two minutes.............finish that to get his second challenge........
Now the second challenge is where this achievement comes in.......he wants you to kill 30 infected with (unlimited) throwing stars in two minutes.............if you are at 0% on this achievement......what you should do is kill 25, then let the time run out, you will be able to restart the challenge, and your past kills will still count towards the cheevo......
The achievement pops after you kill the 50th infected.......easy peasy!!