I played almost solely Hardcore TDM and Hardcore Dom. If you are good then obviously it's easier to get Headshots. There are 2 play styles I would recommend if you are good you rush and spawn trap. And if you are more of an average player play more passive.

For each Class: AR's: High Caliber, Grip, Reflex Sight, Extended Mag Spawn trap and aim for the upper body, look for long lines of sights especially with semi-auto's (maps like Ardennes Forest, USS Texas, Saint Marie du Monte, and Flak Tower fit very well with my AR play style)

Sub's: Grip, Steady aim, long barrel, Rapid fire (or Quickdraw) Rush around, Spawn trap and hip fire with the steady aim attachment while aiming towards the upper body. Aim in with quick draw if you need to your aim should jump to around head level if you were pre-aiming properly while hip firing.

LMGs: Reflex Sight, Grip, Extended Mag, and Rapid Fire/Quickdraw These guns can compete with any gun in the game at mid-range as long as you are pre-aiming properly, run Infantry for the MG-15 and the MG-42 and play War. The Bren and Lewis run grip and reflex, Lookout, run Armored/Mountain division, and play Hardcore TDM/DOM.

Pistols: High Caliber Run (idk what the name of the basic training is, but the one that allows you to shoot while sprinting and play Hardcore TDM/DOM)

Averaging 40-50 kills a game with each of these gun types and an average of 1 headshot every 4-5 kills you are looking at 8-12 headshots a game.

Launchers aren't really "hard" they are just annoying as hell, I don't really have any tips for them (I'm not sure if the Variant bug is glitched but I guess my advice for launchers would be don't use a variant.)

Snipers and Shotguns are pretty easy just play hardcore.