"I’m your huckleberry” means “I’m just the man you’re looking for!”
From The “Historical Dictionary of American Slang” , which has about a third of a column of citations documenting this meaning all through the latter 19th century.
“Dictionary of American Slang: Second Supplemented Edition” (Crowell, 1975):
“A man; specif., the exact kind of man needed for a particular purpose. 1936: “Well, I’m your huckleberry, Mr. Haney.” Tully, “Bruiser,” 37. Since 1880, archaic.
I think it was meant to mean he (Doc) was just a tad bit better than the man he was going to fight.
Does that help?