First, you must complete 4 prior missions before obtaining this one. The quest giver is TK, located in Hallow's End (see video below). For these missions, you must collect brains, which drop from getting head shots off zombies. You will need to collect a grand total of 435 brains to unlock this trophy (250 for the Braaaaaaaaaaaaains! mission alone).
Do not waste time collecting more brains once you have completed one of the missions. For example, if you have collected 50 brains for the third mission and continue to collect brains before turning it in and starting the fourth mission, the extra brains you picked up will not count towards the next mission. So be sure to stop collecting brains once a mission is complete and ready to "turn in."
Thankfully, there are zombies everywhere so there is not a need to really farm one specific area. Just go about progressing through the game and then afterwards, go back and play again if need be.
Here's a good video detailing the jest of the trophy requirements. Credit goes to Double.
Happy Hunting :)