Hello TA!
The simplest way is to build a completely new weapon with "Heavy Elite Frame" as frame, "Surveyor Charge" as Upper Tool and "Directed Ejection Field" as Upper Tool Tip.
Upper tool tip is important because you want to not go into pieces when you shoot your partner.
IMPORTANT: Do NOT use safety guard! It makes this method impossible to use!
I played on Casual difficulty, because you don't get gibbed as easily. I also did this after completing the game.
Trick is this: Have your co-op buddy stand infront of you and shoot his mine launcher into your face or shoulder, maybe a bit over you. All that matters is that his gun points at you. This should bring him to low health at first and on the second shot bring him down. Then mash A asap and have your buddy shoot you again. He'll go down in a single shot even if you have an fully upgraded RIG and therefore this is the easiest solution.
Team damage isn't enabled in the game either, so no danger to the guy who gets the achievement.
I went to the start of Chapter 4 because there are no obstructions at all and as a bonus there is a bench next to you. It took me a single minute to get the achievement, so it is far faster and less risky than getting slashed by necromorphs :)
I have attached an image of the weapon used. I also underlined the most important parts with red. Once again: NO SAFETY GUARD ATTACHMENT!
Your secondary weapon doesn't matter.
Have fun getting downed!