"He's smart. That guy." is not grammatical. The structure is very different from your Batman quote.
Interested in Language
I would like to know if there is a word to describe a manner of speaking, wherein the task, person, and so on, is defined, before the said object is introduced to the listener. Now bear with me in my example, but again I'm curious. The example is a statement from the movie 'The Dark Knight'. I don't know if the statement is grammatically correct (it may not be), but the Joker says: "It's simple. We kill the Batman." Granted that is a response to a question. Is that the key to the answer being grammatically correct? He describe the task to be simple. Then he said what the task was.
Again, I may be misinterpreting what he said. Similarly, I've heard people say: "He's smart. That guy." Is that grammatically correct?
Henchman: "How do we make Gotham City safe for criminals again?"
Joker: "It's simple. We kill the Batman".
What is simple is making Gotham City safe for criminals again. Killing the Batman is the means to that end.