Throw a bucket of ice water over her. Or pour it down her neck suddenly. Autistic people tend to feel senses more extremely.

Although I think you should work towards figuring out a hygiene routine? Maybe a sponge scrub with lukewarm water? Unless you already have a preferred one.

Showering with clothes on may help as the transition is more gradual feeling, but may be seen as unorthodox (although that shouldn’t stop you)