One of the big advantages of an LMG in FFA is that it can have a huge clip size. In TDM, you usually have your allies at your back so ducking down to reload is not as big of a deal, whereas in FFA you have enemies coming from all sides!
One thing that might make a LMG easier to use is a fast secondary, like a pistol, Raffica or Throwing Knife. Shotguns generally take a long time to pull out and since gun-switching with LMG's also takes a while, you only really want to switch in an emergency. At least, that's what I've found.
Also, make sure you use a grip (unless you're in hard core, where you only need to hit one bullet.
RPD is also a great counter to shields, since it works well to spray momentarily exposed appendages.