Here are some tips I can provide since this one was a pain to unlock. Keep in mind this is for the Free-for-All mode.
1. Every man for himself - what makes this so challenging is the fact you have no allies and you're gonna end up in a full lobby almost every time.
2. Take your time - don't go rushing in guns blazing because you can be gunned down from just about anywhere. Move slowly and stay as hidden possible. Normally I'm against camping but you might have to resort to that if you keep dying.
3. Know the maps - each map is designed with lots of hiding spots and close quarters areas so make sure you have the right class for any scenario.
4. Having good gear - The warlord gear is by far your best option. You can regain all your health and armor if your lucky enough to escape being killed. As far as weapons go, just use the weapons you're most comfortable with and aim for the head (obviously).
5. Finally, the best class for this mode I feel is the Engineer. SMG's are great for close quarters encounters (which will happen a lot) and claymores are great for extra kills or those pesky campers.
If Free-for-All isn't working out, then you can try Bag and Tag but just keep these things in mind: There's only one map (I think) and it's huge, you have to pick up player's tags to score, and not many people seem to be playing this mode so you're not likely to even find a match. But if I'm wrong on any of these things, then please tell me.
Well that's pretty much it so if this was helpful in any way, please leave a like or comment.