The key to going positive is based on different factors, for example perks, game type, gun, and attachments.
1. Perks
Use perks that suit the game type and your playstyle. You wouldnt use lightweight if you were an objective/aggressive player because objective get Grenade and Explosives spammed at them (HQ, Demolition, Domination). Use Flak Jacket if you want to secure objective or use Ghost/Blindeye to keep off the radar and flank enemies.
2. Game Type
Find your favourite game type by playing a wide range of modes. Some of my favourites are Domination, Headquarters and Kill Confirmed. Also try to party up with friends and play together as a team.
3. Gun
This is probably the most variable factor that can make you go good in a game or extremely bad. Using SMG's and Shotguns on larger maps like Drone and Carrier will only make you go negative. Using a Sniper Rifle on small maps like Nuketown 2025 is not the best idea to maximise your kills. Use a gun that suits the map and have a wide range of guns in your classes so you will be ready for long range gunfights and close quarters combat.
4 . Attatchments
To many a time have I seen enemies attempt to kill me almost across the map with an SMG with a suppressor. The suppressor attatchment limits the already short range on SMG's. Use attatchments that suit the map and game mode.
Those are just a couple helpful tips ive gotten from playing BO2. Hope they help you out.

As my five classes I have 2 assault rifles, 1 with overkill, 2 SMG's, 1 with overkill and a sniping class.

The guns I use are:

MTAR red dot and silencer.
Overkill gun: DSR 50 with Ballistics.

XM8A1 red dot.

MSMP silencer and foregrip.
Overkill gun: DSR 50 with Ballistics.

MP7 FMJ and forgrip.

Ballista with laser sight.

Maybe try one of the above, I seem to be doing pretty well with them all.

any other classes/other help guys?