Please go read this solution for a definitive version of all the achievements:
Like I said above that solution has a 'more' complete list of what to do. Below I'll list a bunch of stuff that will help you with this achievement. Weapons:
Below I've ranked all the weapons in terms of their effectiveness against enemies (I have not used all these weapons). Primary:
1. SA-58 OSW (Battle Rifle) (Criminals)
2. SCAR-H (Battle Rifle) (Cops)
3. HCAR (Battle Rifle) (Cops)
4. SPAS-12 (Shotgun) (Criminals)
5. HK51 (Battle Rifle) (Criminals)
6. 870P Magnum (Shotgun) (Cops and Criminals)
7. 37 Stakeout (Shotgun) (Cops)
8. Double-Barrel Shotgun (Shotgun) (Cops and Criminals)
The M1A1 is also available for all classes. Secondary:
1. Bald Eagle (Cops and Criminals)
2. M1911A1 (Cops)
3. 45T (Cops and Criminals)
4. .40 Pro (Criminals) Equipment:
Use ammo box, breaching charges and Ballistic shield.
Now for some helpful and insightful tips (these are tips that are exclusively to the Enforcer class, if you want to see more click the link at the top of this solution)
- Use Enforcer's Ballistic shield to completely block your back from enemies firing at you; however avoid pulling it out cause you more often than not will die when you hold it out unfortunately.
- Don't play Enforcer exclusively! Change it up a bit, play a different class from time to time so the grind isn't so boring.
- Im not 100% sure to whether or not vehicle kills count.
- Battle pick ups and equipment kills count towards the achievement
- When you get 1250 kills with any of the guns you unlock the weapon license ability which allows you to use that weapon regardless of what team you're on.
Got anymore tips I can add?
Sorry for any grammatical errors and or mistakes on this guide; I tried my best okay cry If you have any questions regarding this game or any of these achievements hit me up with a message or comment on this solution. Anyway thank you so much for reading and have yourself a good day! Happy Hunting everybody!