This achievement will take the longest to complete but can be done whilst completing other missions. Also if you have more than one player all players can collect towards the total this will make it far easier and quicker.
To activate this misson you have to take TK's wooden leg to his new "zombiefied self" His leg is located on a porch at the south west of the map in Hallows End once you have this in your possesion go into the buiding you found the leg on and give it to zombie TK.
He will then give you the missions to collect zombie brains to feed TK.
Collect 10 Brains
Collect 25 Brains
Collect 50 Brains
Collect 100 Brains
Collect 250 Brains
To get the zombies to drop a brain upon death try to kill them by a head shot. I found using a shotgun and aiming point blank in their face worked best, as opposed to using a sniper rifle.
After collecting each stage you have to return to TK who will eat the brains in front of you and then puke up a collectable as a reward.
Make sure that you collect the set amount for the mission. Return to TK then start the next mission. You cannot collect 200 brains and turn them all in at once.
The best way is use a sniper.