I'm Rank 25. I win more than I lose and have an ok K/D ratio of 1.74. The key is I play with my clan and only with my clan, who I've been playing shooters with for 7 years. We not only communicate well but our playstyles have developed over the years to compliment each other. That's the kind of stuff that's required to do well.
There's no one key tactic that's going to work against all people. Staying grouped works well if the other team isn't communicating and you can mow them down one after the other but if they're grouped too, it's a toss up. If they've tactically set up better with a wider angles and good shots, they'll win out. Reviving down players is also HUGE in terms of gaining parts as well as keeping your pool of lives up. It's a really well designed game in that sense.
There are definitely flaws though in that higher ranked players do have better perks that can contribute to them getting parts faster, not being able to be marked, build faster, etc.. So yeah that's a tough one to get around, though it's not insurmountable.
There are 3 big flaws in this game. Melee, action button priority, and soft geometry corners. Melee is often times very random. Part of this is the unwieldy nature of it and part seems to be Client to Server or Client to Client lag. The best way to make sure you win? Don't Melee unless you've got an upgraded weapon. It's better to try to shoot or run away. Action button priority is just awful. Get the shiv action prompt? Better hope there's not a down enemy, downed ally, or a supply box around you because you'll go for those before you kill your enemy. The last one is the worst. Soft Geometry. What this really amounts to is what you see from your angle isn't what your enemy sees. It results in you getting shot when you think you're in cover. To avoid it.. don't creep out at the edges of objects and walls. Another good rule of thumb is, if you can see your reticle while in cover, some part of you is exposed. I believe the problem is caused by the over the shoulder nature of the cameras.
I enjoy the hell of the MP personally.