An easy way to get this is to carry a weapon of some sort (I was using a spiked bat) and a few things of alcohol (beer, vodka, etc.). I was hitting a survivor with the bat, giving him alcohol, and repeating. After about 3 drinks I unlocked the achievement.
It's also easy to combine this achievement with Puking Rally. Just do this in an area with a lot of zombies and you'll pretty much get it automatically.
Did it on Denyce (the very first survivor) when running on the very first mission to find Frank Zombrex. Before I saved her I just picked up a normal bat from the first maintenance room, then when to the bar in Americana Casino to grab 4 bottles of Vodka. After saving her it was just a matter of hitting her once, giving her a Vodka, and repeat. Achievement unlocked (and some clothes!) after 3 bottles.