Weapons Proficiency are classified into the following categories:
Pistols (Both Repeater & Revolvers)
There are 2 methods to earning the trophy. Keep in mind that whatever weapon you choose to reach level 10 with first, just don't bother playing with any other weapons until you've unlocked the trophy to speed things up.
1) Solo. Since the Pistol Proficiency includes both revolvers and repeaters, using said weapons would be the fastest way to earn the trophy playing solo since they use separate ammunition and will allow you to have multiple options for equipment.
2) Co-op. The best part about this method is that you don't even have to kill anything. Simply equip the weapon you wish to level up and let your group do all work! Even though you aren't doing anything, you still gain proficiency when your group kills enemies.
The choice is yours!
Happy Hunting :)