It would indeed be offensive in that the title would be inaccurate. Epic is the only way to describe so perfectly executed, so brilliantly designed, and so superior to a number of things that exist in our physical world and metaphysical dimension. Manly would illicit the connotation of something...human to describe something inhuman in grandeur.
Daiki, my good sir. Rai-you is just a grumpy moron.
They are not called Manly Rap Battles of History for a reason. Because manly would not describe the magnitude of their superior design. Manly is word that does bring the same connotations of superiority as the word epic has.
Another apt example is Daiki in Years of Lies. He is epic and therefore more awesome than that Ryuu guy.