In China, the American Volunteer Group, US fliers in service to the Chinese government was shooting down Zeros using P-40's. Claire L. Chennault, the group's commander, had figured out ways to take advantage of the Zero's shortcomings, such as its lack of armor. In addition, the Japanese army pilots they were facing were not as well trained as their counterparts in the navy.
The American Volunteer Group was soon absorbed by the US military and eventually became the Fourteenth Air Force.
You mean ki-43's which kind of look the same as the A-6m zero.