The Wasted Garage - Killing the Boss
Go through the newly opened door with your gun out. Just clear the room like you've done before, and look for the stairs down to the main garage. Note that there's some buckshot and fatboy rounds on the main counter. Grab them; you'll need them for the next fight.
As you walk past the cars, you should hear one start and come rushing at you. Get behind the nearby barrels and watch as it wrecks. The Wasted bandits' leader will jump up onto the turret and start firing at you.
Thankfully, that's his only real attack. The gun hurts a lot, and has a nasty knockback effect, but you can avoid it fairly well by hiding behind the barrels. He will occasionally send someone out to rush you. Listen for the combat yells. I suggest that you just switch to the shotgun and nail them when they run past the barrels.
For killing the gunner, there are a few options. Grenades are never a bad idea. You can also switch to the fatboys in your pistol and wait for him to stop firing. The gun jams occasionally, and when it does he raises his head as he pounds on it. It's quite easy to just fire a few shots into his head. Normal rounds don't do a lot, but six of the fatboys should kill him.
You can also look just past the barrels that you should be hiding behind. There's a panel on the pillar to the right that should be flashing. If you hit this, an explosion tank will start moving down the little crane above him. Watch for it to go right over his head and shoot the tank to blow it up and do a whole lot of damage.
Turn to the left and shoot the unfortunate bandit that decides to try to rush you, then move forward. If you go around the wreckage you should be able to find the piston that you need on a table. Grab the gas cans and oil cans around the shop, and look for a “wasted turret" collector card. The last thing of interest is a locked door near the exit.
As you approach the rear garage doors, one man will kick the doors open. Fill him with buckshot to clear the way. If you can make a lock grinder, use it to grind off the lock to the storeroom to get some feltrite crystals, buckshot and a bunch of oil cans.
When you're ready, walk out the side door.
There will be two more bandits standing by their ATV. Two well-placed wingsticks will make it yours. Jump onto it and ride back to the Hagar Settlement and turn in your parts to finish the buggy.
While you're here, you should really sell all the loot that you picked up. Also, you may want to invest in some ammo, especially if you burned through your fatboys and buckshot.
Once the buggy's done, walk toward the dinner. You can pick up a collector card for the grenade if you look at the pipe next to Becky.