how does mercutio describe tybalt? what does he mean by "Prince of th cats" and how does he think romeo will react to the challenge of a duel?

Mercutio knows that Tybalt likes to fight. There is some respect for Tybalt because of his fiery blood. He says Tybalt is also good with words. As for Prince of Cats, check this out:

https://www.gradesaver.com/romeo-and-juliet/q-and-a/why-is-tybalt-called-prince-of-cats-79879/