Very interesting question, and one I do not have the answer for. I'm inclined to guess it definitely predates 1925, and I'm imagining the phrase probably was in use at least dating back to Corbett, but my speculation might be well be off the mark, and perhaps badly so.
I wouldn't want to guess who was the first to use the term, but there's an article here from 1919 by Robert Edgren describing the use of combination punches by Tom Sharkey, Jim Jeffries and others. https://chroniclingamerica.loc.gov/lccn/sn86092536/1919-04-21/ed-1/seq-7/
Saw a 1906 reference in an article about Frankie Neil and Harry Baker: "DeWitt Van Cort taught Harry the triple left jab--three short pokes delivered inside of a second--and this combination punch spilled Frankie's nose and mouth all over his face." Also, from a 1909 article on Billy Papke: "Papke has a combination punch with his right and left. Last May he put Hugo Kelly to sleep in the first round with this double wallop." I saw a few earlier references but they all appeared to apply to combination punch-shoves or punch-grapples.
