I'm at the Boss in the Wasted Garage and I didn't realize the bad guys keep coming at you until you waste the gunner. I'm already low on ammo and I'm down to normal pistol rounds. Short of starting over, looking for tips to snipe the gunner quickly without getting taken out by the machine gun. I've capped him several times from a distance but he won't go down!

Did you notice the yellow fire extinguisher tank type thing when you entered the area? Every few seconds one of those is passed over the gunner on a guide rail above his head. Take that out when it's right over him - hey presto.

Alternatively, get behind the block on the left, take out the latest wasted henchman, then peer over the top and unload rounds into his head, whilst still in cover.

Every once in a while, he takes a break from shooting and stands up.
Also, there's a switch on a pilar across from the corner, but I never got a chance to try that out.

i got as close as possible and used the shotgun
almost completly killed him the first try
just waited until he stopped shooting then finished him
that way i only had to kill 3 of his 'henchman' to kill
i tried first with the pistol and for me that did not work
too many other bandits to kill as they seem to come non stop

If you blow up the gas tank and it doesn't kill the gunner. Flick the switch and a new tank is coming up.

When you hear the gun stop shooting, and the gunner starts banging on the machine gun to unjam it--he is fully exposed for some head shots. The gun jams alot, just back up to a save spot, kills the spawns and wait for the next jam to step out and head shot him.
A few extra bandages help too. Found it easy once I realised what the banging sound was when the machine gun isn't shooting.

I ended up re-playing the whole level since I screwed it up the first time. I didn't realize the monocular doubles as a zoom for your pistol...duh! So the second time I loaded up my pistol w/ some fatboys and killed the gunner right away using the monocular scope.

there's a easy way to kill him: 3 frag grenades. don't belive me? try it your self