I suggest:

She barely hit me on the back.

She hit me lightly on the back.

The use of “hardly” is often used as a qualifier like “almost.”

Also, play around with placing the adverb. For “barely” I think it sounds better after the subject. For “lightly” I like it better in the middle.

You can also be a bit more creative with it and say:

She tapped me on the back.

She hit my back, but I hardly felt it. (note the use of hardly as a qualifier here)

Naturally, if you were looking to make “hard” into the adverb, then you don’t want to say “lightly” or barely.” I suggest adverbs such as “heavily,” “mightily,” or “squarely.”

If “hardly” is supposed to mean ‘barely’ or ‘slightly’, then it’s also technically correct, but very awkward, and unclear what the meaning is. I would use the word “slightly” instead, and/or rearrange the sentence to ” She hit me on the back slightly” or “She hit me on the back, but just slightly”.

Hardly doesn’t make sense in the sentence.

It would be correct if you worded it as,

She hit me hard on the back.

or

She hardly hit me on my back.

Depending on what you are saying.

Another correct, but awkward, example would be.

She hardly hit me on the back.

This example is awkward and would probably sound better as,

She barely hit me on the back.

One thing you should strive to do is try and limit your prepositions or dangling a participle.

Wheras,

She hardly hit me on the back.

A better choice would be,

She barely hit my back.

or

She hit my back but only barely.

she hit me hard on the back..

If you are trying to say that this person struck you very lightly, I would say: “She barely hit me on the back”.

If you are trying to say this person struck you with great force, I would say: “She hit me, hard, on the back”.

but if it is present you should add ‘s’