To easy farm this achievement play the first two levels on whatever difficulty you like, with a second player. The second player is for more enemies in the opening area, so you don't have to go through the tutorial. Eat everything you see before the zombies you create do; on the second level instead of getting into the hovercar, fart and eat the two cops closest to you and follow the line of humans walking away from you. If done correctly, they won't spot you and you can kill plenty of humans without getting noticed. Just go through the entire level and repeat this process and go back to the first level. Easy enemy and civilian brains, and even just playing the entire game gets you a bunch of brains but the easiest farm for this achievement would be this strat.