Several people have commented on not being able to get "One with the Shadows" or said that it's glitchy. I think the description in the guide is just incomplete; you don't need to physically hide at all, you just need to go somewhere unobserved and switch costumes. I'm sure hiding doesn't hurt, but if you don't switch costumes then the achievement will not pop.
So do it the way the guide says, but remember to put on the outfit of one of the guys you killed. That is why the achievement pops.
Another easy way to do it is in Chinatown; go into the alley by the tattoo parlor, kill the cop and cause a general ruckus then run upstairs and shoot the drug dealer. Put on his outfit and stand around for 15 secs and the achievement will pop.