The Combat Shotgun in Call of Duty WW2 is a pump action shotgun. It has good range and high damage output. However, at anything other then close range, you will not have a chance against any other class of gun. This means that you need to maneuver the map to seek out clusters of enemies in close quarters. Whilst killing enemies you cannot avoid but being noticed so you need to be quick about dispatching enemies and be aware that they will come after you. The best attachments for the Combat Shotgun are those that help you deal with more enemies. The extended mag attachment increases the number of shells you can shoot between reloads. The advanced rifling Combat Shotgun attachment increases the range which helps you deal with those enemies at the edge of your range.
The best division for the Combat Shotgun is expeditionary because it gives you incendiary shells which help increase the amount of damage that the Combat Shotgun does. The best basic training for the Combat Shotgun is hustle which lets you reload faster and reload whilst sprinting, which is especially important because after each death you have to reload the incendiary shells. Pick your favorite secondary in case you run out of ammo and use the mk 2 fragmentation grenade to attack objectives. Use the smoke grenade to screen your movement through open areas and help confuse bunches of enemies by throwing the smoke at them. Here is the complete best class setup for the Combat Shotgun.
As found on Youtube