I had recently started to use Hand Cannons when I realized something interesting: that their accuracy allows them to shoot off a Phalanx's head through his shield. I did this by simply stepping to the left of the Phalanx, you know the side with his hand sticking out, and if you look closely you can see his helmet. Aim for it and you can oneshot a Phalanx without shooting his arm, just his head. This may be unintentional but his head kind of peeks out from behind his shield (maybe so he can see). This can also be completed using a Scout Rifle (so I am told) and a sniper Rifle, though the hand cannon is much more effective at this.