Hey guys, me and my friend, Nitram1711 found a really good strategy to get this achievement which I thought I'd share.
This is probably the final achievement you will attain from the Coop Mode which requires you to unlock 7 guns by accumulating a score of 441,000.
EDIT: With the new R3 Patch, this total score needed is a mere 160,000. Disappointing for people that did the grinding but this should make it a lot easier for people going for it. If you already had 160,000xp previously, you'll automatically get the unlock if you start a coop game. Thanks to BloodngunsMrphy, KM55D & Grimmr74 for letting me know about this!
EDIT: The shortcut DLC DOES NOT unlock this achievement. You still need the points. Thanks Osiris for confirming this!
MP412 REX: 5,000pts. Revolver Handgun (All Kits)
KH-2002: 17,000pts. Burst Fire Assault Rifle (Assault Kit)
MP7: 34,000pts. High Fire Rate PDW (All Kits)
M39 EMR: 58,000pts. Standard Battle Rifle (Recon Kit)
93R: 86,000pts. Burst Fire Handgun (All Kits)
SG553: 120,000pts. Standard Carbine (Engineer Kit)
G3A3: 160,000pts. 20rd Magazine Assault Rifle (Assault Kit)
Presuming you are going for the other Coop Mode achievements (including the mission specific ones and the Hard difficulty one), you will probably have run through the missions a few times and accumulated anywhere between 200-300K.
To effectively farm points for the rest, select the final mission, The Eleventh Hour, on easy. On the first hallway of the mission, kill all the PLR and have one player pick up the AK-74m with the Thermal Scope. Continue the mission until you have to disarm the bomb, and have the player drop the gun for a random gun before defusing. When the bomb is defused, the player's guns are reset back to the M16A3 ACOG and the Shotgun. Pick up the AK again and look nearby for the ammo crate and there should be another AK that the other player can pick up.
Work your way through the mission until just before you exit the Metro and you will notice a resupply crate. Remember this as you will be backtracking to it several times. Move to the police cars outside the stock exchange and have one sit on the cars to the left and one to the right. Switch the AK to Single Fire mode, (Press down on the D-Pad) and there will be an endless stream of enemies.
Each kill is worth 100 points, a Headshot is worth 10 and a spot (using the Back button) is worth 10. If you sit outside here for about 30mins you should get about 40,000-50,000. As mentioned by DisguisedMike, the EA servers have been a bit off lately and it's a good idea to move through the mission or bleed yourself out as this will save your progress as opposed to one of you dropping out.
EDIT: If you prefer to simply replay the missions to obtain the remaining points, the most effective mission is Hit and Run. The mission is quite quick on Easy, and there are many enemies. Thanks to Lc Moog for this input. From his experience, you can get 11,000-12,000 points every time you complete the mission in about 5-6 minutes.
Definately worth picking up these guns for multiplayer. I alternate between the 93R and the REX for my handguns since unlocking them. The REX is quite powerful and the Raffica is deadly accurate and I love the three round burst.
The M39 EMR is definately a good choice for those who use the MK11 MOD or the SVD as it can be used by both factions. While they unlock for the opposite teams at later levels, it's worthwhile to use the EMR as you can unlock attachments quicker!
The MP7 is also a great gun in close quarters especially when coupled with Extended Mags which double the clip size to 40+1.
Thanks to my pal, Nitram1711 for helping me with all of the Coop missions and for coming up with the idea to farm points at the Stock Exchange.
Best of Luck!