To unlock the Weapon Aficionado trophy in Borderlands: Game of the Year Edition, you will need to reach proficiency level 10 with any of the different weapon types.
To do this naturally, you'll have to stick to one type of weapon (Assault Rifle, SMG, Pistol, Sniper etc). As you kill enemies, your weapon proficiency with that weapon type will begin to rise. If you stick to one type of weapon, the trophy will unlock on your way to reaching level 50.
You can also unlock this quite easily if you join a high-level friends game. Let your friend do all the shooting, and all you will need to do is stand back and watch the weapon proficiency level of whichever gun you have equipped climb pretty quickly. In under an hour using this method, you will unlock the trophy.