Japan is simultaneously easy and hard.
Easy: your grand strategy is to attack everything everywhere. Rd 1: invade Soviet Far East. If the Russian player holds, use your superior air force and local land units to destroy Soviet super stack – big help to Germany, no 18 inf rolling to Moscow. Rd2-4, build up inf and trns for seizure of south sea islands Rnd 4. Start placing miICs Rd3, first in Ksu, then Hon and then Fic. With those working you should build enough stuff to take out India Rnd 7/8 depending on how competent your opponent is. Then wheel around and take out Hawaii for win.
Hard: You have to be very careful when moving your ships around to safeguard them from counter attack by allies. Especially watch the US opening a block that Anzac can exploit. Be sure to have enough dds to block US fleets from knocking your fleet out, especially blocking them from moving into sz6. If the US goes for KJF, these problems are magnified by an order of 10.