How to use an enemy as a meatbag?
Solution 1:
The only way I've noticed it works for me is when I have a enemy that you can usually counter with B, and then hit X to do an assassination kill.
When the other guards are about to fire, and you are near one of the easy guards that I described above, look at the HUD on the bottom right, at certain times you will see the A button show different text for you to grab someone for a shield.