HUMANKIND

Pretty straightforward: do not get your units killed wil in a war. A unit consists of multiple soldiers: you can lose a soldier for this achievement, but not the entire unit.

My advice is to set the game on the lowest difficulty, 1 ai-player as beginners, choose a militaristic culture as soon as possible and make sure any Civic, tenet of event outcome you pick will give you more power to your combat units.

But you also have to play aggressive, because of war score. War score is accumulated by having demands onder other players (stole my independent people, following a different religion, claimed territory next to my border, attack my units, etc.). Always demand those things in the Crisis tab on the empire screen. That way your war score will go up. You want to get this as quick as possible, so you must expand as quick as possible to get as much interaction with the enemy empire as you can.

As a militaristic culture, you should already start with a base score of 80 instead of 50, meaning you can go to war at all times, but my advice is to wait until your war score is at 100. Preferably, your war score is higher than theirs.

Now, move your units as close to their borders as you can, declare war and fight tactical to save your units. A unit is almost dead? Move it around and defend it with other units circled around it.

The reason you want to do this as quick as possible at the beginning of the game is because your enemy probably does not have ranged units yet, meaning you can really defend your damaged units well.
Plus attacking soon is smart because the cities are not that big yet, the enemy empire will not have that much armies yet and they are scattered over the map exploring and the cities levies/defences are not strong yet.

Once their war score reaches zero you will be notified, pick all the demands you want and end the war. The achievement should pop.