How to climb above diamond in warfare mode ?

Hello guys.

I'm writing this post for any advices because it's became very hard to climb recently. It was so easy until diamond rank, and there is a massive difference with the operation mode, where you need at max rank over one kill to maintain yourself in the God of war rank.

I know it's not supposed to be easy and i read somewhere that i need at 750+ SPM to climb to Marshall. However I can't understand how it's possible.

I usually try to play every class when needed, but also tried full medic, Sineva spam shield, sniper classes, black guy ult and nades on choke points, I rarely get over 700 at the end of the game.

What am I missing ? Do i need three premades ? Am i supposed to play tanks or heli ? Spamming vehicules to success or what ?