"Boss shot dead his girl friend." Please elaborate the structure of the sentence. Can I replace "shot" with any other word, like stabbed, poisoned or beat?
According to Google's Ngrams, stabbed, poisoned, beat, and beaten dead are all virtually non-existent, compared to shot dead. While they are all equally grammatically correct, only shot dead is idiomatic.
As to what part of speech hard is in this instance, I suspect (but cannot say authoritatively) that it's acting as an adverb. Other, similar constructions would include:
He was hit hard.
She ran quickly.
He spoke softly.