Slimes, magma cubes, and ender dragons (charge attack) can hit every tick, meaning they can break your shield in seconds.
Steps to reproduce:
- Trap a large or medium slime, or a magma cube of any size, in a small area with walls and a roof so that it cannot jump to full height or be knocked back by the shield the full distance it normally would be.
- Enter the area while holding a shield. (See Shield breaker.mp4.)
- Trap a large or medium slime, or a magma cube of any size, in a minecart.
- Use a shield while walking into the minecart. (See Shield breaker 2.mp4.)
- Use a shield while the ender dragon charges at you. (See Shield breaker 3.mp4.)
Slimes, magma cubes, and the ender dragon attack the same number of times per second as other melee attack mobs.
These attacks have no attack/damage cooldown. The shield blocks the attack so many times per second that it breaks even with full durability.