The other two solutions for this achievement are absolutely right, but I think there is another and much faster way to get this out of the way within just a few seconds.
You need to unlock a universal skin, e.g. the elephant skin or the different color skins which you can find below every single gun in the game (within the camo-menue).
Now, instead of just equipping the camo on your gun, press "view fullscreen". This will allow you to enable the option "equip to all" in the bottom left corner of the screen. Now you have even more than 30 weapons with a camo on it and the achievement should unlock.
The only downside of this is that you'll have to change every gun camo manually if you want something else on it later. But this is indeed the fastest way to earn the achievement.