Learn Russian: Она потрясающая актриса.

You would say Что за потрясающая актриса! Because что за is followed by the nominative case, the phrase потрясающая актриса stays exactly the same as it is in this sentence. If you want to include the pronoun, you can say Что она за потрясающая актриса!

The word потрясающая looks like a verb form. What is its origin?

It comes from the verb потрясать (to shake, to astound). While it originated as an active present participle meaning 'astounding', it is now fully adapted as an everyday adjective meaning 'stunning', 'amazing', or 'mind-blowing'.

Why isn't there a dash between the subject and the predicate here, since the verb 'to be' is omitted?

A dash is generally omitted when the subject is a personal pronoun like Она, even if the core predicate is a noun (актриса). If the subject were a regular noun instead, you would typically write the dash: Мария — потрясающая актриса.

Does it change the meaning if I say Она актриса потрясающая?

Yes, placing the adjective after the noun shifts the focus and gives it much stronger emotional emphasis. You might use that inverted word order to passionately contradict someone who doubted her talent, emphasizing that she is an actress who is truly stunning.