Minecraft wiki entry for Wither_cage, associated text: Damaging the wither will cause it to break all the blocks in its hit box, including obsidian. This is why bedrock was used to trap the wither. The wither can be damaged in several ways, including getting hit by any projectile, explosions, suffocation. Suffocation and being hit by a projectile such as a snowball are the most common methods of harnessing this destructive ability. In fact, placing two indestructible end crystals can be placed in front of the two side heads, which cause the skulls’ explosions to damage the wither. Suffocation can cause problems because the pistons that push the blocks into the wither’s suffocation line could end up pushing the wither out of the cage.