XCOM: Enemy Unknown

As the description suggests, this only applies to a crash site mission, not simply any mission done solo. While choosing squad members for the mission, simply remove all but one squad member, and start the mission.

Obviously, this is far easier when assaulting one of the Small Scout ships with just 6-7 aliens, rather than one of the Supply Barge, Abductor, or Battleships. Even late in the game, you'll have opportunities to shoot down the Small Scout crafts, so you can be choosy.

Your play style will dictate what type of soldier you bring for this effort; I chose a Support Soldier, as I was trying this pretty early in the campaign, and the added speed (Corporal -- sprinter perk) comes in handy. However, trying this later in the campaign certainly has its advantages, if you have the Ghost Armor, which can bring out the advantages of every class.

Always remember to save often, so you can simply re-load, should something unfortunate happen, and you can choose a different course of action.

If you feel like knocking out two birds with one stone, make your solo soldier a female, so you can also get:

You can easily get this achievement during the tutorial when you first shoot down a small alien craft. There are two groups of two sectoids and one crystaline alien. Try to go around the edge of the map so as to activate one group at a time. In my case, I used a heavy corporal - Fire rocket and bullet swarm - equipped with a grenade. Like the others said, take it slow.

This achievement gets easier the longer you wait:

1. I would suggest using a Heavy Colonel - Fire Rocket/ Bullet Swarm/ Shredder Rocket/ HEAT ammo/ Will to Survive/ Danger Zone/ Mayhem
2. Ghost armor is a very big help.
3. Blaster Launcher and Heavy Plasma is also a very big help.
4. The smaller the ship the easier, small Scout ships have between 5-7 aliens.
5. It must be a UFO mission you shot down via. Interceptor/ Firestorm
6. If an alien is successful using Mind Control on your only/ last soldier you automaticaly lose the mission.
7. If you have the Hyperwave Relay you can see what aliens are on the UFO.

Once you have found the right size ship you can keep reloading (before you shot it down) and it will create a new random assortment of aliens, keep doing this till you find the ones you prefer.
I kept loading till I recieved some weaker Thin Men, Sectoid Commanders and Cyberdisk groups (to take advantage of Heat ammo). Since I had Thin Men i equipped a medkit to make me immune to poison but it was unnecessary.

The idea is to NOT let the aliens even have a turn, keep on ghost armor till you find a group:
If you find a group of 1 or 2- ghost between them and use Bullet Swarm (depending on their strength).
If you find a group of 3 or more- back up and fire a Rocket.

You can easily just do it on the very first ufo you shoot down. I sent a assault trooper in making sure to save at the beginning of each turn. You must utilize full cover whenever possible, as well as make use of your run & gun skill to get you close to single aliens. I was able to take out all of the aliens alone using this method. Also, just make sure to shoot any sectoids that are using mind merge on another one. That will kill both of them in one shot. Save your grenade for whenever 2 sectoids are grouped together or for the outsider thats located in the ship.

This guide is meant for people like me who didn't win this achievement during one of your full playthroughs, and you want to try to go back and get it with the least work.

Start a game in Easy difficulty. On your first mission, there are four sectoids and four soldiers. Try to let each soldier get one kill, and try to avoid getting shot. Ideally, you will end the first mission with four healthy squaddies. In my case, I ended it with two healthy squaddies, one wounded squaddie, and a healthy rookie.

Now continue playing and start your next mission. Take any healthy squaddies you have, then any healthy rookies from your first mission, then fill the squad out with new rookies. This mission has six sectoids. Once again try to let everyone on your squad get at least one kill, and try not to let them get hurt. You're trying to get as many soldiers promoted in your first two missions as possible, without getting hurt.

When the second mission is over, switch to Classic difficulty and SAVE. *This* save will be the one you need to reload if you don't get the achievement on your next try. Now wait for the first UFO and shoot it down. Look at your soldiers and pick the one you like the best. If you have a Corporal, that's probably who you should take. Otherwise, you're looking for a Squaddie with high aim and a tactical class you feel comfortable with. Personally, I prefer the Heavy for this achievement because of the rocket. You may like the Support for the smoke or the Assault for the Run-N-Gun. I don't feel like the Sniper is the way to go, because Headshot doesn't gain you anything against the weak enemies you'll be facing.

Pick your one soldier and launch the mission. Proceed in small moves, staying within the light blue line and moving from full cover to full cover as much as possible, and ending your turn with Overwatch. There will be two groups of two sectoids each, outside the ship, and an Outsider inside. So, you'll probably have three little battles to fight. Reload your weapon after each battle, but always make sure you're in full cover when reloading. If you're like me and used the Heavy, you can use the rocket on the Outsider, who tends to stay in full cover and can't be killed with a frag grenade. Use the frag grenade on a sectoid who is doing mind merge with another one, killing two birds with one stone. (Speaking of birds, why are there so many birds soaring *through* the UFO?)

If you die, Load Game from before you shot down the UFO. There are a lot of comments on the other solutions from people who didn't get the achievement even though they thought they did everything they were supposed to. I was the same way; I must have won this battle seven times without unlocking the achievement. In my experience, you won't get it if you Load Game at any time after you make contact with the UFO in Mission Control. You can't change difficulties after that either.

The achievement will unlock at the "Mission Completed!" screen that comes up after the last enemy is killed.

Getting the achivement this way took me about an hour, but that included a lot of fails where I loaded from the wrong point. I reckon it will take 30-45 minutes if you load from the right spot.

I did this very late game against a supply barge - 12 aliens (2 x Sectopod; 4 x Drone; 3 x Muton; 2 x Elite and 1 x Ethereal). Key was using a Psi Heavy - Bullet Swarm; 3 rockets; heat ammo - with all psi abilities - equipped with psi armor; heavy plasma; blaster launcher (doesn't need line of sight) and mind shield. This set up gives a 100% chance of mind control against all enemies except Ethereals - 'will' was 125.

Basically take it slow - very slow! Get some luck - mind control whenver possible as these become preferential targets for the aliens - and retreat when necessary for MC to recharge. Sectopods go down in one bullet swarm - you need to be close and this depends on enemy location. Ideally they spawn behind the first doors you open.

When you have 1 Elite and the Ethereal left, MC the Elite - next turn move the Elite first to get an attack on the Ethereal - this is important as you need to reduce the Ethereal's HP enough that two rockets can kill it. Fire 1st rocket - hitting the Elite doesn't matter BUT you must not kill it - repeat you must not kill it. The Ethereal will attack and kill the Elite next round leaving you to fire another rocket for the kill and the achievement.

I did this with no reloads as I'd heard rumours that these disable the achievement - although not personally confirmed.

I did this based on a combo of the above ideas. At the very end of my second playthrough on Easy, before activating the Gallop chamber I switched difficulty to Classic and kept scanning for the smallest UFO I could find.

IMPORTANT - you must change difficulty to Classic BEFORE scanning for the UFO.
The first time I attempted this I changed difficulty to Classic after the UFO was shot down and it never popped. Grrrr.

The smallest UFO I got was a medium Scout with 8 Aliens encountered in this order -
- Cyberdisk with 2 supporting drones
- 3x Muton Elite
- Muton Elite + Ethereal

So 2 waves of three and the boss fight with a single muton.

Choice of soldier was Heavy Colonel with Bullet Swarm, HEAT, and a Plasma Blaster Launcher. Ghost Armour helped a lot with the Muton Elites. I took grenades but never used them, probably better off with a mind shield in hindsight.

Strategy was basically move into full cover gradually, overwatch and let them come to me. This worked for the Cyberdisk which was taken out with HEAT rocket in one go.

The group of Mutons I ghosted next to and then did the bullet swarm thing before they even had a chance to move as someone else suggested. Once they were done that just left one muton guarding the Ethereal, luckily he came to me so was sorted with Overwatch,

The way I dealt with the Ethereal was firing "blind" with the plasma launcher which was the only way I could take the Ethereal down without closing in for Line of sight and getting mind controlled.

I fired my remaining 2 rockets using indirect fire from outside the UFO through a gap in the roof where I roughly guessed he was, and it did the trick. Satisfying seeing that "16 damage critical hit pop up" both times!