Jack is (hard/hardly) a man to be afraid of .Какой правильный.

 Jack is a hard man to be afraid of.

We use hard because with nouns we use adjectives and not adverbs as hardly.

E.g.  Lucy is a serious girl to speak to  such nonsense.

      But for example : He acts awfully. Here we use adverb because with the verb we use adverbs.