Minecraft wiki entry for Advancement_guide, associated text: Then you need to find a skeleton, a pillager, a stray, a shulker, a blaze, a drowned with a trident, another player to help you, or anything else that fires projectiles. Holding the shield in either hand, hold use, and let the projectile shoot toward you. The shield should block it (except if an arrow was shot from a crossbow with Piercing), therefore rewarding you the advancement.