Editor mod / Server / virtual arsenal for the explosives shells, equip a huge plate carrier on you and the entities will have the same, shoot at them with 30mm HE rounds / 20mm, whatever. Incredible resistance.
Virtual Arsenal for the Helmet/Headshot problem, still I think that's only a virtual arsenal bug, but i'm not sure.
Again editor / server / VA for the heart/lungs shot problem, when the bullet is not hitting against the armor and should pierce his arm to finish in the body, deadly shot.