KanjiJLPT N1
Shell

husk, nut shell

Mnemonic

A gentleman with superfluous confidence grabs a tomahawk and charges at a giant crab. One mighty swing cracks the beast's thick shell wide open, revealing gleaming treasure hidden inside!
Additional thoughts
Picture a well-dressed gentleman, absurdly overconfident, wielding a tomahawk against an enormous crab. The key image is the dramatic crack of the shell shattering under the blow — hard exterior giving way.
Quick recall
A gentleman with superfluous bravado smashes a crab's shell open with a tomahawk.

Details

The keyword for 殻 is shell. This kanji refers to a hard outer covering or casing, such as the shell of an egg, a nut, or a crustacean. It conveys the idea of a protective exterior layer that encloses something within. By extension, it can also be used figuratively to describe breaking out of one's shell or shedding an old form.
On'yomi
かく、こく、ばい
Kun'yomi
から、がら