XCOM: Enemy Unknown

To locate the aliens base, you'll need to capture an outsider alive. Once you research the outsider shard it turned into, you'll get a cut scene showing the location of the base.

On around the 6th mission in Germany UFO Crash you will run into a certain type of alien. Its the 3rd/4th type you encounter and it appears from within the crashed ship. What I did is made sure that all of my characters except my sniper had the arc thrower. (This is the device that you will eventually be able to research and then build through the engineer) Just make sure you equip it to your soldiers by clicking loadout and scrolling down to where your grenade slot is and switching to it. This mission begins with a 2d battle between your spaceship and the UFO so just press X and you should shoot it down. Don't attempt to shoot the outsider and make sure that when you are getting close that you will only be taking 1 move and having the circle pop up to be able to stun him once you move. Finish off the rest or you can stun them as well. Research the outsider shard and once its finished researching achievement should pop.