The Giant Shelly is an enemy found in the Cavern layer. It walks around slowly, and occasionally curls into its shell. It then starts spinning and hurls itself at the player, gaining increased defense and dealing high contact damage. Giant Shellies attack very quickly and deal high damage for the stage in the game where they are found.
Giant Shellies are unique in that their ability to spawn is connected to two other enemies: the Crawdad and the Salamander. Of these three enemies, a world will choose two types that it can spawn, leaving the third inaccessible. Giant Shellies will never spawn in a world that features both Crawdads and Salamanders together. In the Desktop version, Console version, Mobile version, and Nintendo Switch version, this is taken into account by the Bestiary progression logic, and the missing enemy will register as unlocked when the other two exclusive creatures are unlocked.
The Giant Shelly is very similar to the Giant Tortoise and Ice Tortoise. The cooldown between its spin attacks is 6.67 seconds by default. It is reset if the Giant Shelly takes damage.
If the Giant Shelly is more than 12.5 tiles away from the player and has an unobstructed line of sight to them, the cooldown is reduced to 2.22 seconds.
If it is more than 37.5 tiles away, and has an unobstructed line of sight or is no more than 12.5 tiles above them, it is further reduced to 1.33 seconds.
Each spin attack consists of one second of retracting into the shell, spinning for 1.5 seconds, then decelerating until coming to a stop on a solid surface, and coming out of the shell for one second again. Attacking a Giant Shelly at any time cancels the spin attack.
If in water when ending a spin attack, the Giant Shelly will use the spin attack again immediately.
Although the trajectory of the Giant Shelly during its spin attack is determined the moment it retracts into its shell and is based on the player's position at that time, the enemy does recognize the player's current position, and will slow down very quickly and attempt to drop down as soon as it is above the player.
Any weapon with a high rate of attack is effective, as it prevents the Giant Shelly from using its spin attack.
Melee weapons are useful as long as the player stays close to the Giant Shelly. Even when not attacking continuously, the spin attack cooldown is much higher at close range.
Giant Shellies cannot drop through platforms. Setting up several rows of long platforms significantly reduces the danger of the spin attacks.
Like their aforementioned analogs, Crawdads and Salamanders, Giant Shellies have multiple colored variants. Also like said analogs, these differently colored variants are statistically identical, unlike various Zombie varieties meaning that both colored variants share the same Bestiary entry.
The Bestiary entry for the Giant Shelly: "To overcome their slow speed, shellies have developed the ability to hide inside their spiked shell and hurl themselves at their foes."