This achievement along with every other weapon achievement is obtained by grinding out kills on dinos. Each weapon has 3 achievements, those being: Apprentice, Master and Expert. (100, 500 and 1000 kills)
These kills are cumulative across all gameplay and game modes, but with this method you can grind those kills much, much quicker.
To start, you will require the workshop map "
XP Grind 1.2" To enable workshop content in the game, you will need to find Orion: Prelude in your games library, right click it, select properties, click the Beta tab and then select the sdkbeta. This will enable workshop content as well as implement a few other upgrades the developers introduced in the sdk beta. (Weapon audio is noticeably better.)
Now create a private lobby and select the XP Grind 1.2 map. Set the difficulty to Prehistoric to unlock another table of settings that affect the EXP Modifier. You can play around with this to get the settings you want, but I suggest following these settings for optimal credit/exp gain. You will level up fast and have more than enough credits to buy any and everything.
Recommended game settings:
Game Mode: Survival
Map: XPGRIND12
Difficulty: Prehistoric
Dino Health: 25%
Dino Damage: 400%
Generator Timer: 1 min
Credit Drop Rate: 300%
Small Dino Ratio: 500%
Large Dino Ratio: 500%
Vehicles: Disabled
Starting Credits: 0
Silly Waves: Enabled
With these settings your exp modifier should be over 1000%
Once the match starts simply activate the generator to begin the round. There are loot crates floating in the air that you can open to get some starting credits. Once the dinosaurs drop though, you'll have hundreds of thousands of credits within minutes. Buy any weapon you wish to earn the achievements for and simply unload on the trapped Raptors that spawn endlessly. you can get about 50-100 kills a minute depending on what weapon you're grinding with.
You can use this method and map to go for many achievements other than weapons such as Loot Crates, Exp, credits and purchases, etc. Here is a video for a visual guide on what to expect.