KanjiJLPT N2
Undress

undress, removing, escape from, get rid of, be left out, take off

Mnemonic

Under the moon, a demon begins to undress, shedding its scaly skin layer by layer. Each piece of armor-like hide falls away, revealing something unexpectedly human beneath. The moon's pale light watches as the demon peels off its final disguise—to undress is to break free.
Additional thoughts
Picture a moonlit clearing where a horned demon strips away layers of dark, reptilian hide. The moon (flesh/body radical) plus demon together evoke the act of shedding or removing. Each discarded layer reinforces the idea of escaping a cursed form.
Quick recall
A demon sheds its cursed skin under the moon, Undressing to finally break free.

Details

The keyword for 脱 is undress. This kanji conveys the act of removing or taking off clothing, shoes, or other coverings from the body. By extension, it also carries meanings of escaping, breaking free, or shedding something, much as one sheds a layer of clothing. It can further imply omission or leaving something out, as in dropping or missing a character in writing.
On'yomi
だつ
Kun'yomi
ぬ.ぐ、ぬ.げる