Army of Two

Carrier 1 is the best map for this. The 2nd section (the one past the two catwalks on either side of the beginning with 3 barrels each) you can do a cycle where you are essentially continually destroying barrels. There's about 5 on the ground, the same 5 on the other side, then proceed to the end and their will be two up above on ledges around the corner. Do the same 2 on the other side, then go back to the start of the area and the first ones will have respawned. Rinse & repeat.

What we did was had one guy (call him P1) doing this area repeatedly. While he was doing this, another (call him P2) was doing the 8 barrels in the beginning area (with the catwalks). And a third (call him P3) was in another area later in the level (our P4 already had the acheivement).

Once P1 got his achievement, P2 then took over destroying barrels in that area. By the time P2 got his, P3 also had gotten his. Note that the achievement pops up immediately mid-match, you don't have to wait until the end of the match for it.

Only took an hour or so for us to get all 4 SSC achievements, and that was with a guy dropping out partway through.

Carrier 1 map works great, we left red team P1 up front killing the quick respawning 8 barrels (3 up top and 1 below each side) and we sent P2 on red team behind that covering all the way to team lift wall. For blue team we cleared all the way through. P1 of blue team stayed below the circular silo part destroying all barrels around the area while P2 on blue team stood at the end shooting everything in sight. I thought the backside worked very well with almost no movement, but the front 2 people got thiers first. In the end in a single game maxing 15 minute rounds, we were done with all 500 barrels on all 4 people in 29 minutes.
I suggest going for the ranked win, all stages won on 3 round game and the 75% accuracy on this map to start, then finish with barrels once you are familiar. Can do entire SSC expansion with 4 people in an hour.
Goodluck and enjoy everyone

Warehouse 2 works excellent if you do it one at a time. I cant remember who I was playing with that figured this out (sorry bud) but have everyone work their way to the end and stop at the glass ramp that leads to the bridge and the finale. There are plenty of barrels at the end leading to the finish. If everyone stays at the bottom of the ramp then one person can shoot the barrels.

If you stay right at the top of the ramp and come around the corner just enough to shoot the barrels you can then turn around to head back down the ramp and the barrels will respawn instantly. You do not need to go down the ramp but just to the top of it before you can swing back around and peer around the corner to see barrels popping back up.

We took turns letting one person get the achievement at a time and we each got it in one SSC challenge. Remember you only have 15 min so dont screw around and race to the barrels so you will have a lot of time to shoot at them.