I have played a couple of long campaigns from the Japanese side and one aborted Nat vs Com pbem. I am no expert though.
The Japanese are a foot army thus are quite sensitive to terrain especially when the number of turns forces a straight advance. Look for obvious advance routes and target them for arty. Japanese platoons are pretty well rounded but the different weapon systems move at different rates so work on separating the GLs and scouts from the main body.
As China, troop quality differences should give you a numbers advantage. Those conscripts won't last long but they will whittle away at the Japanese as they advance - think of them as claymore mines rather than maneuver units- lock their range down to 1 or 2 and don't move them. Armour is not really worth it but cavalry can give the Chinese superior movement capability.
Japanese AA in this period is also pretty poor - if you get any are consider buying an observation unit (there seems to be an endless supply of disaffected whiskey soaked American pilots willing to fly for the right price) and circle it over your base area - maybe out as far as the centre line - unless mule mounted, the Japanese AA is sloooow.
As for the Chinese advance - I really don't have any suggestions.
If playing PBEM - I would suggested you ask your opponent to buy the GL as grenade launcher units rather than the GL as mortar units - this cuts their effectiveness quite a bit.