Super easy, believe it or not.
First, here's the layout I used:
Level: The Beach
Chassis: Selker Mk. II, Sprinter legs (to make your scrap collecting faster & easier)
Weapons: None! Just to be safe, no accidental discharge. (If ya get what I'm sayin'.....)
Emplacements: Advanced Shotgun Turret, Advanced Machine Gun Turret (Didn't even use this), Makeshift Sniper Turret, Cluster Mine Layer
You could probably get away with just the Advanced Shotgun if you wanted to, but using the Shotgun, Sniper, and Mine Layer, I got through the level with 90% health (enough for a gold medal, as well.) You could also probably switch out the chassis and bring along the magnet support turret to make your job a little easier.
Here's how you'll want to set up. The first thing you do is place a shotgun at the very front of the boat. This is important to clean up all the little stragglers, as they will most likely make it past your first line of defense now and then. The next shotgun you will place will be in front of the boat, on the rounded concrete area. After that, place one to the right on the rounded concrete in the back, to keep the little guys from flanking you.
Your job will be to just run around collecting scrap to build more turrets as often as possible. When you can, place a mine layer in the middle section next to the shotgun turret in front. Behind this and to the right (not too far back) place a sniper. This guy will be the one who does most of the killing on Big Willy when he shows up, so make sure that it's far enough forward to only barely reach the boat in the back.
After you've got these 5 turrets set up, just upgrade your shotguns when you can. Run around collecting scrap, and soon enough you'll be hearing the sweet, sweet sound of an achievement pop!