ROTK Appendix A does say
Quote:
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Boromir was a great captain, and even the Witch-king feared him. He was noble and fair of face, a man strong in body and in will...
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.
Though Boromir I did defeat the uruks, it isn't said that he ever 'confronted' the Witch-king or any of the Nazgûl
personally.
That the WK 'feared' him could mean simply that he had special worry about Boromir as a double threat; a powerful field-leader and a wise and respected ruler.
I don't think based upon that that it could be said Boromir I was 'one of the greatest warriors Elf or Human', though.