The Worst is ostensibly completely indestructible on the outside of his body; he is able to withstand heavy physical attacks due to his soft, blob-like body.
The Worst has malleable skin, allowing him to absorb any physical impacts.
The Worst can survive energy attacks, such as Exo-Skull's laser.
The Worst is resistant against extreme heat and immune to lava.
The Worst is naturally resistant to poisons.