Instructions
Select "Create a Class" from the online multiplayer lobby menu and equip a primary weapon with a high accuracy and damage rating, as well as a decent rate of fire. The weapons class needed for these types of stats is "Assault Riffles." The FAMAS rifle is the most accurate of the fully automatic guns, while the M-16 has the highest accuracy and damage rating of the semi-automatics.
Equip a gunsight. Using an attachment such as the "Red Dot Sight" will allow you greater visual accuracy when zeroing in on an enemy's head.
Enter a round and pause the game. From the "Pause" menu, access "Controller Options." Make sure "Auto-Aim Assist" is turned on -- this will cause you to lock onto a target faster when using the "Aim/Look" trigger.
Select "Control Sensitivity" and increase it from its preset "Medium" setting. While it might take a few rounds to get used to the touchy controls, you will be able to aim more quickly along the X and Y axes for faster headshots.