To start off with, find a nice quiet conquest server (On End-Game)
Try to get a server with no-more than eight spaces, and invite as many of your friends as possible.

When the game starts, make sure that you are the one to drive the IFV out of the C130, then just kill one of your friends when their not in a vehicle with the air-dropped IFV, one of your friends in an IFV which was NOT airdropped (With the airdropped IFV)and an enemy car (Agian, with the air-dropped IFV). That's the hardest of the achievements required to get this gun out of the way.

Next, hop into one of the anti-air vehicles (The converted humvees) and get your friends to hop into the choppers, make sure they dont jump out, getting the kill whilst their inside the helicopter is always the safest bet.

And finally, Go into an empty server with your freinds, and capture the enemy flag 3 times, then allow them to take your flag away from its spawning location and then kill them and return the flag 5 times.

You now have the S-TAC!

(If i have forgotten something, or done something wrong :S, please tell, and i'll change it!)

*Im not saying this is the best way to get the S-TAC, all im saying is that this was the way i got it, and i found it real easy.

*If you dont have any friends, post in the forums and let people know you are wanting the S-TAC and i'm sure someone will arrange something with you.