You can obtain the rebel achievement by killing 20 adult brains. Completing this will take you 20% of the way towards the master achievement.

Master Rebel Achievement

Kill 100 adult brains.


Note that mobs generated by spawners do not count toward this achievement.

There are a number of worlds with dedicated brain farms.

Be aware that killing more than 8 adult brains in a short amount of time will result in their comrades coming to take revenge by spawning right on top of your head.