Enemy at the Gates Achievement

*This is an online achievement*
 
Opposite from Opposing Force (15G), this one requires you to kill the invading sniper before they kill you. In order for someone to invade your game, you must allow Axis Invasions in your game. To turn it on or off, select Options from the main menu, then under the ‘Game’ heading you can toggle it. This can also be done while in a mission from the pause menu. This cannot be boosted with a friend; you must play against randoms.
 
Note: It is suggested that you allow cross play for a better connection to potential invaders. Turning it on will allow people with a different Xbox console or Windows 10 to join your game. It is in the same menu as ‘Allow Axis Invasion’.
 
While you’re playing a mission, if someone joins your game you, you will get a notification on screen that you have been invaded. Throughout the invasion, there will be a notification in the top-left corner of the screen. Now it’s just a matter of locating and killing the invader. As with the other invasion achievement, you can use phones to determine their general location. Also, watch for enemies that move differently from the A.I. And, as with Opposing Force (15G), camping too long will result in your death. Once you kill the invader with any weapon, the achievement will unlock.
 
If you are playing in co-op, the person that kills the invader gets the achievement.