XCOM 2

I waited until I had a Spark unit at Champion level with the NOVA ability. You could always use grenades but I find I'm usually out by the missions end....

Wait until you have a main plot or non-time sensitive mission - hold off doing it until another mission appears e.g. guerilla ops, retaliation etc then do the time sensitive mission. Once you're down to mopping up the last few critters, trigger NOVA multiple times until you get down below half health.

Then go do the other mission & make sure it survives. Or if not able to do the next mission straightaway for whatever reason, make sure you go to Engineering & remove your damaged Spark from the repair list. They do seem to still 'heal' over time (don't ask me how...) but if you're quick you should still be below half health when you undertake the second mission

Unlike human soldiers spark units can go on missions while injured/damaged. After a unit is damaged in battle, and the health is under, half bring it on another mission. Keep it in the back and let your other units do the fighting to avoid further damage. As long as it survives and you complete the mission you will get the achievement.