I was in a pug 25 man naxx, and we could not get him down because we had the most terrible kiters known to man.
A random dps told us that we could bug out gluth so that you fight him inside the tunnel you run through to get to him, and zombies can not reach him because they can't run up walls.
So what you need to do in order to achieve this, is have your tank and 1-2 healers run to the VERY EDGE of the tunnel, without falling off.
This will aggro gluth, and sort of bug him out.(you'll see him kind of bouncing up and down). It takes about 30 seconds for him to bug out into the tunnel, but its well worth the wait if you're in a terrible pug
this may take 1 or 2 tries, because the tank will fall off or something, but its the easiest way to kill this guy. It also allows for easier decimate healing since everyone is grouped up.