Achievement Difficulty - 1 / 5
Estimated Time Span from Fresh Start : 30 Mins
You can earn any mastery for a weapon by playing against "Easy Bots" in offline deathmatch mode. They present the weakest challenge although it may be possible to find idle players and / or fast spawns in community maps.
Also consider using the smallest map as that produces the most spawns.
I have not yet tested if community maps qualify for achievements, but since offline bots are so easy and straightforward, this method is satisfactory for getting this achievement.