I am glad 1ke posted that link, giving me an occasion to quickly read Jude again: Where are you seeing this “defense against the alternative teachings and accusations that Jesus was but a mere man”? Rather, the letter is mainly about sexual promiscuity.
It seems to me you are leaping to this interpretation from verse 4, “godless persons, who pervert the grace of our God into licentiousness and who deny our only Master and Lord, Jesus Christ”. There is nothing here about anyone claiming that Jesus was merely human. There are many ways to ‘deny’ someone; to teach that Jesus was merely human is very specific and not justified by the text: Usually, when the Bible uses ‘deny’ in this way, the meaning is disobedience, disregard, i.e. a rejection of authority or rebellion. Hence Jesus saying “deny oneself and follow me”, meaning that we must reject our disordered passions, rebel against our natural (carnal) desires to follow instead a supernatural goal, etc. So this verse means ‘denying the authority of Jesus’; it does not mean ‘denying that Jesus was divine’: You need more from the text to arrive at this more specific conclusion.
Please clarify, especially if I have overlooked something.