This is easier than it sounds. You can't let the enemy with the pipe that alerts everyone see you. If you hear him bang his pipe then you have to restart that area.
I found it easier going left and killing off most enemies there by luring. The gang leader will go in a counter-clockwise direction around the level so he's fairly easy to keep track of. I headed to the top right and waited on the corner to take him out without him seeing me.