To get this achievement, you need to build the shield and use the shield blast (RT) on a downed teammate.
The shield blast can only be used after charging up your key with 3 zombie souls. To do this, pull out your shield, and hold (RT) on a zombie to collect it’s soul. Your key will be glowing blue after this is done 3 times. Now you’re ready to unlock the achievement. QUICK AND EASY:
Enter a lobby SOLO
on casual, add some bots and start the game.
Now progress through the map until you have the shield built and power turned on.
Now go near a trap (with your teammates nearby) and activate the trap. 9/10 times the bot is going to go down from the trap. Use the shield blast on your downed teammate and voila the achievement is yours.
Don’t know where the shield parts are? Watch this video by MrRoflWaffles below.