Shellda's Level Up Ability is similar to the Kickstarter Exclusive Hellcow's Ability, albeit, the main difference is that Shellda's ability can be activated every turn upon activation instead of one, but can only move two rooms at a time, unlike Hellcow, who can move around all the rooms in a dungeon.
Shellda's Ability is best activated as soon as possible as it allows a player to either adapt their dungeon to changing situations (like needing to place a room that is early in the dungeon to later in the dungeon). At the same time time, it also allows the player to mess with an opponent's dungeon, thus messing up any planned strategy the opponent may have had.
The artwork and name suggest this Boss is a cross between Zelda and Bowser, of the Legend of Zelda series and the Super Mario series. The artwork even features stone pipes on the edges of the portrait.