Since neither guide actually tell you the requirements for this one, you need 40 kills in cover and 100 lancer kills as a Soldier. You can do this on solo starting from wave 1 on any map. I used Relic on Casual and setup the fabricator at the western side of the map(use back, then b to see the map).
It really doesn't matter what skill cards you use.
Basically set up camp in the far corner in cover and kill any enemies that come your way. Feel free to go out in the open when there are only a few enemies left, you don't need to stay in cover all the time. Try to stick to using the Lancer to get this over quickly, but if you feel like mixing it up against certain enemies that's fine, just don't get rid of the Lancer.
This popped at the end of wave 6 for me, but could take longer for you based on enemy set and cover usage