Strategy for Operation Watchtower
Operation Watchtower Strategy for Japan
Playing in Normal/Normal.
This is no picnic. Your first instinct might be to rush the troops you have on Rabaul onto the transports and do an amphibious assault on Guadalcanal right away. This will most likely lead to you ending up fighting a losing battle against the Allied fleet, which right now is much stronger than the Imperial fleet.
Instead, go off and hide your fleet (New Ireland is the easiest and most obvious choice) until you get your first reinforcements. When that happens, you can finally fight the Allied fleet on an equal footing.
As soon as you can win naval battles from the allies, take control of the sea. Now you might be thinking it's best to attack Guadalcanal. But it's not. Start taking the other islands in the south. And as soon as you've captured 2-3 islands, the AI will get nervous and start attacking random islands of you. But to attack you, they will use their troops from Guadalcanal.
So from the moment you see they are attacking an island, you know that Guadalcanal is empty. Send at least 4 divisions to do an amphibious assault. If however the allies are winning the battle for the island they are attacking, just send you're fleet to the boats, and hunt them away. The troops will return to Guadalcanal now, but now they have already lost a lot of organization. So it should be easy to defeat them.
When Guadalcanal is in your hands, all you can now do, is capture the remaining 1-2 islands. And wait during the last three months, while you fleet keeps sinking ships of the poor allies who don't have even the smallest port left.