Armor Piercing Defender is a gear perk that improves your defense against both armor-piercing rounds and blasts. It covers two of the most dangerous damage types the FND throw at you.
Armor-piercing rifle fire and explosions both hurt you less. That covers heavy troops, snipers and grenade spam in one slot.
Wear one of the gear pieces that carry it: Cyber-Jacket, Rioter Vest MK II. Each piece only takes one body slot, so plan around the other perks you want.
Yes for loud, frontline play. Two defense types in one perk is good value for a chest slot.