Unfortunately this achievement is glitched.
After fully levelling all 18 in game weapons the achievement didn't unlock. I then managed to get hold of a DlC code from the Collectors Edition which gives the Quatermain Pietta to use in MP. I then levelled this gun up and again no achievement.
I've emailed Nordic Games about the glitch but havant received a reply. If anyone wants to contact them here are their contact details.
http://www.nordicgames.at/index.php/contactShould they ever fix it I found the best way to level the guns for a party of two to be as follows.
Use Map Anubis, Capture the Artifact ( Reason I uses this as it's 3 consecutive rounds with no loading or trying to get the right map though any team based mode will do)
The person who is going to be killed first needs to host so if playing CTA mode they will die in rounds 1 & 3 switching roles in round 2.
The person getting the kills now needs to go to the area pictured below which is found to the right of the Fire statue up on the raised wooden walkway.
The player going for the kills needs to position them selves up against the barrier to the right so you end up with a view like this.
Have the dieing player grenade themselves and they will continually spawn in the area pictured right in front of you providing you stay position behind the barrier as shown. Again this only works with two players in the game as soon as more players are introduced they start spawning elsewhere.
Where possible try to have both players carry the same load outs for ammo refills, with the primary weapons you can get up to 120/130 kills a round, shotguns and pistols are a little slower as you will have to slightly move the dieing player a little forward for kills but you can still easily get 70/80 kills a round for those weapons.