Strange Brigade

This is story related and cannot be missed.

After overcoming the gauntlet of the undead, the spikes at the end of the hallway will drop and you will enter another chamber. Climb down the steps to the Sacred Sanctuary. At the back of the chamber is a glowing blue orb and there are two beams you need to focus on two gems on either side of that orb. The first beam is just to the left of the orb. The second one is on the right on the upper level.

Once you’ve collected the souls of the shield, Bes will say that the Second Damned has awoken. Once again, you need to stand your ground and defeat them. When you've reached a certain point, the bells hanging from the ceiling will toll and the Protector of the Shield, which is the same boss as in 'The Towering Temple' level in the main game, will appear.

To defeat it, you need to damage it by shooting its armor away to reveal a glowing blue orb on its torso. To do damage to the Protector, you need to shoot the blue orb. After you damage it enough, it will teleport to a safe zone and shoots an energy beam, so try to avoid the beam. Continue to shoot the blue orb until it's defeated. You will then be free to collect the shield piece which will unlock the achievement.