Like A GloveG 30

For this achievement you must get 5 kills with the arc attachment for 12 different guns. Arcs can be found by a dead soldier in corridors you can follow. You can always find one in the first area; which has three paths to find one along with the other two having item schematics(no more toolboxes. They will be on the floor.) Also, be warned, each area will either spawn a rhino or a mammoth(upgraded rhino which is blue and can call in reinforcements).

An easy way to get this is to buy the attachment carry over perk in the armory upgrade section. It costs 10 teeth to purchase. This will also save you from having to find new ones or having to swap the attachment, buy a new gun and run back and re-equip the ark. If you don't have this upgrade and you buy a new gun, you will lose the ark attachment.

If you are decent doing extinction, I recommend doing this achievement in solo mode. The mammoths and rhino take less bullets to kill them( they guard your only chance to conquer the arc attachment). That and less enemies and the like. Also, I will attempt a video to showcase all weapon locations. Or if I can, will write explaining things if I can make it brief and not all that confusing.

Now for the 12 guns needed; Also added from memory how much each gun was. If a weapon was hard to find, I stated where to find it.
Maverick- $1500
Ameli- $3000
Chain Saw - $3000
Remington R5 - $2000
VKS - $1500
K7 - $2000
IA-2 - $2000 (It's located under the pink glowing mushroom looking things in the middle of the first area)
Mk14 - $2000
Ripper -$1500 ( Will be found by the blockade leading to the second area. Look to the right by the vanguard post and it's in the bushes touching the wall. )
Tac-12 - $2000
USR - $1500 (Can only find it from the second area and thereafter. You'll find it on the wall next to the vanguard laptop leading to area three)
Bulldog - $2500 (Can only find it starting from area two.)

*Note- The kasket and pistol are not needed and cannot equip the arc attachment. In order for you to safely unlock it, get a few extra kills with each weapon just in case. Must be completed in one game. Will not work across multiple playthroughs