Not really a "guide" so to speak, I'll delete this when/if someone wants to create a more complete version. However, this is just a heads up that if you run the tutorial mission, the soldier deaths will ruin the chance of getting this achievement. So make sure you skip the tutorial before you start down the road for this one.
As discussed in this thread of the XCom 2 forum:The Untouchables - Tutorial DeathsBLUEBERRYPETE
I bought the DLC, and my SPARK (non human robot) squad member died on the last mission and I still got the achievement.
So there may be some leeway given in the final mission, or regarding SPARK units when playing with the DLC. Ideally some other people can confirm the particulars of this.
OK, so confirmed by Lt Davo
that the final mission is the key and not simply the unit being a SPARK.Orange1155
has confirmed that losing mind controlled enemies during the final battle does not void the achievement, but the lost "soldiers" do show up in the Bar's roster. I would assume mind controlled enemies that are lost at any point in the game should not invalidate this achievement, although I would wait for a confirmation from someone before taking that chance.
(Seriously, someone post a proper solution, this looks like garbage)