This is my Guide
1. Use the best weapon you are best at, (recommended is Assault Rifles), and conserve ammo, (unless u carry 3 of the same gun like me)
2. Stick together like glue, in groups of 2-8, however sometimes being a loner is better if your team gets you in trouble (like spawns enemies, and brings them to you, or standing in front of you when you are firing a LAW).
3. Dont go afk much, or play badly, and dont play too serious, play easy but good.
4. Avoid standing close to a enemy (because of grenades).
5. Do not charge, (unless you are me, i seem to be able to do that well strangely, and i only lose 50 hp...hehe)
This guide might help, i use M4A1s(multiple in fact, don't run out of ammo) and a LAW if possible.
I finished it like 5 times over, and my best go had 99 kills, and i was the last one, and i had to fight for ages to finish it, (might have been hard, forgot) MUHHAAHAHA, rofl.
Best weapon on my opinion, M4A1(and variants) for headshotters and sprayers, as you can go full auto, and do straight head shots. Ak-47 and G36E for those accurate guys who use burst fire, and smgs, (mp5 best in my opinion) for sprayers, and finally snipers like L96, dragunov, sr-25 and similar. This is only my opinion, so if you are better with other things, then go with it.