“Phalanx” are a rank of Cabal.
"Phalanx are shield-carrying infantry that act as a mobile defensive front, holding ground or pushing forward at will. Their shields can withstand most conventional firearms. Flank them, or you’re just wasting ammo."
Shield and Hand Cannon
A mobile defensive front. You need to flank them to have any chance of getting past the shield. Another strategy is to shoot the phalanx's exposed hand (the one with the gun), which can deal mild damage even when the shield is raised. Deal enough damage this way to cause the phalanx to recoil in pain, briefly lowering his shield in the process and opening up an opportunity for a headshot/bodyshot. They will either push forward or hold their ground, creating a wall of shields.