This achievement requires you to create the secretive 'Wingman' item, by combining a Nectar (mixed drink), with a Bee (obtained from unique zombies) in a MAINTAINANCE ROOM.
(I wanted to specify that so you won't make the mistake I did of wasting time trying to combine them in a blender. Silly me.)
Nectar is created by gathering two Orange Juice's, or one Orange Juice with one Beer, and combing them in a blender. (There are other recipes for Nectar, but these two are easiest).
Bee's are collected from unique zombies, immediately after Case File: "Zombrex 2", on Day 2 (September 26) between 7:00am-8:00am. Unique zombies have red lumps on their necks, and often look as though they are trying to tear/ scratch/ rip it out.
***You do not need to complete case files to trigger Bee's, just the two Zombrex shots for Katey.
With both items in your inventory, head to the nearest Maintainence Room (check your map by pushing select, and Maintainence Rooms appear as blue squares with wrench icon), and combine the two items to create "Wingman", and unlock this easy achievement for 20 points!
Congrats! Wingman, when smashed, releases a bee that will follow Chuck, attract/ distract nearby zombies, and kill them for bonus PP!