Got this one in co-op with my buddy ShadyShall last night on Jungle Heat. We tried lil devilz method a few times, but always got overrun during the last 2 waves. The method that worked for us was a slight variation on a Gamespot YouTube video by Justin Calvert (linked below). All credit goes to him.
Basically, you use every emplacement. Here was the setup we used (same as the video).
1. Place your Artillary on the big square in the back.
2. Place a machine gun on the circle next to the Toybox entrance on the back right. This is the only unit you need to upgrade.
3. Place 4 Anti-tank Rockets. All four go next to the main road leading to your Toybox, two in the front and two along the left side of the road.
4. Place a Makeshift on the last spot, on the left closest to the rock.
5. Place several pieces of barbwire on the road, right in front of the Toybox entrance (click down on left thumbstick, rotate with RB). This will slow down infantry if they walk through it.
Your build order should be Artillary first, followed by the machine gun and then two Anti-tank rockets. The last two Anti-tank and Makeshift can be built whenever. As far as upgrades, only upgrade the machine gun. The video shows him doing it with only one upgrade. We kept having issues with one or two soldiers getting through on the last infantry wave, so we upgraded it to level 3. The key to the whole thing, as previously stated on the other solutions, is to keep rotating your artillary to where the action is. Also, the two front Anti-tank Rockets will take the brunt of the damage, so repair them when needed. Also, don't forget to hit x to bring in the next wave when prompted to help keep your mission time down. Don't forget that the Final Wave has no infantry, so feel free to sell the Makeshift and machine gun if you are a little short on money. When the achevement popped for us, we actually had not sold anything back, so you you do not have to do that part if you feel you have enpugh money for gold. Basically, it you just follow the video, you should have your achievement in a few tries.