T'Challa with a month of a prep is a bit extreme, IMO; I think he would decisely take down Mandarin with that much prep. I think giving then evenly dispersed prep time would make it more even.
The Mandarin would win. Panther is skilled, Mandarin is a superhuman martial artist. Panther is just now learning some low-level magics, The Mandarin mystically turned back history in China. Panther's tech is good, The Mandarin has improved on the tech of an alien civilization. The Mandarin is what Panther wants to be when he grows up.