Hint: lower difficulty to casual and get familiar with the biplane's machine guns & slow speed control (right stick back). If you're quick you can machine gun numerous infantry as they gather in their trench, but you'll get most of your kills by hitting the infantry as they come down the hill. Approach them slow and get what you can from the front, then circle around and pick off any remaining as they cross the bridge.
When the armoured infantry come a marching bombs are your best bet (crash landings don't hurt either) then the air planes are a fun dogfight. But don't get sidetracked on units in small groups when you've got to be ready for 30+ infantry.
This was my favourite challenge and it really turned me on to the power of the biplane combos!