slap her hard!

Hi! Kindly correct this sentece!

She has totally gone mad. The only thing left is to slap her hard to bring her back to her senses.


I would probably change the first sentence:
She has gone totally mad (she has gone completely mad).

“He slapped her in/on her face.”
Which preposition is appropriate?

Both can be used. Both are appropriate. Both are often interchangeable.

‘She got a slap in the face’ - she got hit in the face (though it can also be used metaphorically).