Here's the only 2 things you need to worry about this achievement:
1- Go play the main mission and side quests all the way through. You will earn enough OC to buy all the weapons you need to purchase for this achievement. That doesn't include the 100,000 OC weapon which, therefore, you DON'T need for it.
2- Each time you want to level up a weapon that is not your favorite to keep playing with, go find a Chaos Squad (multiplayer online) match. It goes MUCH FASTER than using it in the campaign. You don't have to use the respective weapon during all of the Chaos Squad minigames, only at the final round, when you have to defend your Vats from the enemy. Use all the ammo you've got for the weapon that you're leveling, then go after more ammo if necessary and keep using it. That way you will level up said weapon in a matter of 5 minutes or less, if you use it against a big wave of monters.
If the simplicity of this solution helped you, please leave a thumbs up to let me know. Thanks!