The White Knight shield will appear on the field of battle when the Black Knight spawns. The Black Knight spawns at random and will attack everyone and everything on the battlefield. Just after the Black Knight spawns a floating white shield will spawn and you need to move your 'Rally Troop' over the white shield to pick it up. This will give you immunity to the Black Knights attacks.
You can unlock this during a quick match and have a second controller connected to make this achievement extremely easy.
Pick up 50 of these shields and the achievement is yours.