To get this trophy, you need to build the Spectral Shield and use the shield blast (press L1 then hold L2 and press R2) on a downed teammate.
In order to use your shield blast you will need to charge your Spectral Shield by using your key to collect 3 souls. To do this, pull out your shield (L1) and hold R2 on a zombie to collect it’s soul. After collecting the 3rd soul your key will be glowing blue and you can now use your shield blast.
Guide to build the Spectral Shield: