As I wrote earlier, Chiang Kai-shek did pretty much everything in his power to not engage with the Japanese army when at all possible. That went for the areas and troops he controlled or was loosely allied with. Generallismo Cash My Check as he was affectionately known as by Roosevelt and most US leaders basically blackmailed the US for big heaping chunks of cash by threatening to make a seperate peace with Japan. Thus, he did tie up the Kwantung Army and who knows how many Japanese resources.
The communists fought a pretty good guerilla engagement but the areas where the communists were strongest were generally in the central and northwest interior of China - impoverished areas far away from the plum eastern seaboard.
The Japanese didn’t suffer any great defeats that I can think of off hand with the Chinese. Someone please chime in if you’ve got a good example.