As said in the above solution. The achievement is for mastery for 3 weapons. Not all of them.
Mastery challenges are given once a weapon has all 12 upgrades installed. Upgrade kits can be obtained as collectables in the campaign. Or given for ranking up.
Here are the 3 easiest weapons and where to get all 3 done in under an hour:MP44
- Get a kill every 2 seconds for a total of 30 kills with the MP44.M30 DRILLING
- Kill 250 zombies by dismembering limbs with the M30 Drilling.THOMPSON
- Shoot and dismember the legs of 200 zombies with the Thompson.The level and method:
You'll want to load the level 'All roads lead to Hell'. On chapter 4 'Hitler's welcome party. I had it set to 'cadet' with '1 player' zombies.
Once the mission loads up, get the gun you want to get mastery for and make your way out the saferoom. Walk to the right where you'll see a ton of zombies just standing around waiting to be killed. You'll see a red explosive barrel. Shoot the barrel next to all the zombies which will cause a chain reaction, netting you a 30-40 combo. The explosion from the barrel will dismember a bunch of limbs giving you mastery with ease.
Once the progress bar on the left comes up for your mastery challenge. Press start and restart checkpoint. Then repeat the step of blowing up the barrel next to the zombies and you'll get your mastery in no time. Sometimes you may need to wait a minute for your progress bar to pop up.
Here is a clip of me doing the Thompson for example:
The MP44 is by far the easiest here as you only need to shoot the barrel one time and that will pop the mastery. That's if you hit over a 30+ combo.
The other 2 weapons can be done exactly the same with a few reloaded checkpoints.