Kill 53,596 infected.

Infected can be any of the Hunter forms and also the infected civilians. If you are trying to boost the achievement, it is better to wait until the city has been approximately 60-80% infected as they will be everywhere. Simply run around killing everything you see as quickly as possible. It will also help if you use infected vision, which will make all infected enemies grow red.

The quickest way to do this is replay the event "Rolling Thunder" (located in the SW corner of the map) which has you using the Thermobolic Tank to kill as many infected as possible. You can easily get 750+ in only a few minutes each time you play this event.