I actually just found a way to idle this on accident. Not that it's really needed. I was doing as solutions stated above, Dead Center - Chapter 2 (Streets). Play up to when you have to retrieve the cola, open the store door, but don't go in, just stand outside and kill the waves of zombies, group with your AI, and they will protect you. I was playing Campaign, instead of Singleplayer, so when I paused to go do something else, I was expecting to come back dead, but my health was still in green, and saw like 5 Boomer Bile containers lying around on the ground, I went around and picked one up, threw it, and picked up the next, etc. I went to check my achievement progress and I had went from 0/10 to 5/10 during the 10 min. period I was gone. Then I healed myself, and idled the rest.