There are 7 weapon types that you can level up, you level up by having a weapon out whilst enemies are killed, both by yourself and by people in co-op. The weapon types are:
◾Pistols
◾Submachine Guns
◾Combat Rifles
◾Shotguns
◾Sniper rifles
◾Rocket Launchers
◾Eridian Weapons
To make sure you get this easily try to concentrate on one weapon type, pistol will be the easiest as you get more of them but you can use whatever type of weapon you want. I personally got it on shotgun.
You can check what proficiency level you are at for each type by pressing
and scrolling to the 'Character' tab using
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