XCOM 2

No casualties means no deaths - injuries are ok.

You should definitely do this on easy (and no ironman), and also keep a save from when you are on the avenger, right before going out on this mission. Sending troops that are tired has a high risk of them developing negative traits (fear of a certain enemy, obsessive reloader that wastes an action, etc)

You might get lucky and be able to do this in a cheesy way very early on. For me, right after the early mission "A New Alliance", only one of my soldiers was tired, but he was tired for a good 14 days. You have a higher chance of tired soldiers if you do the first 3 missions on a higher difficulty level (thanks Lt Davo)

Soon after that, you get your first "Retaliation Strike" mission. I set the difficulty to easy, and I had my one tired soldier run the mission solo (dismissing every other soldier at the planning phase). It is a super easy mission - they may warn you that the Chosen will mess you up, but that wasn't the case for me (both on the run where I sent the tired solo guy, and when I reran it with my full squad after).

The resistance fighters do most of the work for you. I lobbed 1 grenade, and shot one alien, and the resistance fighters killed everyone else. It's a super easy mission to solo, tired soldier or not.

But, even if you can't do it this early, about a month (in-game time) in, you'll have enough back to back missions. If you send the same troops out on them again and again, some will get tired. You'll get to a point where you can send at least 3, maybe 4 troops, all tired. Do so, even if it means leaving a troop slot empty at the planning phase. They'll be fine on the mission, and on easy, you can definitely beat missions short handed. You'll get the achievement at the end of the mission, and then you can reload your original save to avoid the negative traits.

Just a quick note to add to the solutions above...

If you have the bonus for a random ally (Advent or Resistance), this temporary extra unit will cancel this achievement. If you are starting a mission with all tired soldiers but you have a random "bonus" troop, load your auto save from the Geoscape just before the mission and try again.

I didn't get around to trying to do this until late game. I discovered its very easy to do. The requirements for the achievement is sending out a squad of all tired soldiers and completing it with no deaths. As it turns out, you can have a squad of just 1 person!

Easiest way to do this is to send out a tired Reaper on an Advent facility mission. You can easily get to the place to plant the bomb without alerting any enemies. Be aware once you plant the bomb, your concealment will be broken and all enemies become hostile. Best idea is to move your reaper directly on top of the square the game wants you to go and end your turn. This is so you have full movement once you place the bomb. Once bomb is placed, put the dropzone as close to your reaper as possible, move them in and evacuate.

This can be done within 5 minutes with or without an annoying chosen spawning in. Just be sure to save before you make a single move just in case it all goes wrong.

If your reaper(s) are all fully rested and you're impatient, a single reaper can breeze through an advent facility mission. It takes 2 of these missions for a colonel level reaper to be tired. The 3rd mission I used to get this achievement was the ADVENT radio tower mission that goes right into the final mission. That is another easy mission a reaper can solo with stealth and ends the moment you hack the terminal.

this next part is if you somehow are not familiar with how to play the game and you keep losing concealment and don't know why. You will lose concealment the following 4 ways:
1: you attack
2: your soldier is spotted by an enemy
3: you move your soldier and he jumps through a window
4: you move your soldier and he kicks through a door

2 is why you are using a reaper. enemies normally will not see reapers unless they stand right next to them.

3 can be avoided if you hold the A button down to mark what path you want your soldier to go so they don't bust through a window.

4 can be avoided if you move your soldier right outside a door and select the command to open it (its the icon that looks like a button)

If you're keen to do this early on, this can be done fairly easily on any difficulty on the first rescue mission. You'll know its the right mission when you've completed the Lost and Abandoned Mission and can only take a reduced squad. To access the mission you need to do the relevant covert action and then the mission will appear.

By then you should have 1-3 soldiers who are tired from the first few missions that you can take. If you can take a tired specialist that may make it even easier.

The plan on the mission is to stealth through the entire mission up until you pick-up the body/open the door. Watch the patrols carefully and for the stationary targets you can run right through the middle as long as you don't step into an alert square. With a specialist there is a chance you can hack the door to the cell and not change the base alert level.

As soon as you pick-up your recue target the base will go to high alert. If you're prepared (which hopefully you should be reading this) you can base all your troops with enough move space left and be able to place a Skyranger just outside the door and evac on the same turn as picking up the rescue agent. This will also help you unlock: