UPDATE May 18 2017
This technique was not working for awhile. It worked perfectly as shown when I posted the video, but then at some point the enemies were changed. Whether it works depends on the enemies that show up. The method still works if the enemies are Shock Trackers and PeaceMakers. If there are Juvies instead of PeaceMakers then the technique won't work (and in fact I could not complete the round even with level 4 sniper strike). Credit to YouTube user Buzznut3000 for pointing out to me that it was not working. I tested it today and again it works sometimes depending on what enemies show up.
END OF UPDATE
Credits are earned by playing multiplayer or horde mode. To get this achievement you need to earn 100,000 credits. Below is my method for farming credits.
How to Credit Farm with Sniper Strike at Level 2 in Gears of War 4:
Other credit farming guides I've seen have noted using Sniper Strike leveled up to level 4, or level 3 with needing to mop up a few enemies. Since It's a pain to get the skill leveled up I came up with a way to do it with sniper strike at level 2. Each round takes approx 3-4 minutes total, nets 71 credits and 3-4k experience for the profile performing the sniper strikes. You can have other players in the room as well who are away from controller and they too will get the same number of credits. They get much lower experience as they are not getting the ribbons that the sniper strike person gets. If you have online players in the room they should have wired controllers if they are away from controller so their controllers do not disconnect.
Horde Mode with the following settings:
Class: Sniper, must have sniper strike equipped and it must be at least level 2
When round starts run to fabricator. Pick it up, immediately drop it, and immediately pick it up again. Carry to the spot shown in the video. Build and place a barrier as in the video to keep shock trackers out. Then use sniper strike to eliminate enemies (you should be able to use the skill 8 times). There are 39 enemies in total in this wave, so the 8 uses of sniper strike usually have no problem killing all the enemies. On VERY rare occasions you may have to mop up an enemy or two, but that is rare. Once the wave 41 completion screen is finished and wave 42 starts, immediately exit to lobby. As soon as the coalition symbol goes away hit A (skip), then B (close), and finally X (start match) to start the next round.
A video of me performing the technique is below.
And a video of me performing the technique after my update to the solution (video made on May 18 2017)