For the M-COM defender ribbon I mostly used C4. Spawn as support, C4 the objective you plan to defend, and then prone/crouch behind cover in a good defensive position where you can deploy your bipod and get a good view of the M-COM through your scope. If necessary, C4 the cover around the M-COM first to give you a better view. Once you see an enemy standing next to the M-COM, blow your C4 immediately. As a backup you can just blast them with the 200 rounds in your LMG. Squad defend orders on that M-COM are not required AFAIK (I'm pretty sure I didn't use them when I got it).
If you're playing on PS3, be aware that when you get your final ribbon the entire game will freeze for a few seconds before a trophy pops up. When I got my final ribbon (the M-COM defender ribbon) I thought that the game had crashed and nearly put my fist through the TV
EDIT: The outdoor M-COMs are obviously the best for this. I got mine on Grand Bazaar at the first A (one kill - I proned in the corner on the outside of the map closest to their spawn), second B (two kills - I crouched at the high ledge looking down towards B on the defender spawn side), and second A (one kill - this one I hid in a garage near the objective and just popped out when I noticed a teammate turn into a skull near the objective).
EDIT 2: I don't know if this is really all that useful but here's where I mean: