Quickly build up a sizable naval force including a few shogun gunship to take out the soviet factory, shipyard and barracks to stop their attacks. Build a dojo and capture the remaining soviet buildings to gain access to all soviet tech including MiGs and the vacuum imploder.
Build up your base and forces. It is recommended to have a large naval/air force. Sell/Destroy the key soviet buildings when ready.
You will be given 3 fortresses. Back them up with your forces and take out all the enemy shipyards, grinder and construction yards along their shore.
Empire's own superweapon will now be unlocked.
Shortly after, allied forces will invade an take over the south east corner with the ore mines. They can be steamrolled as soon as they arrive. They do start with a sizable force however.
Once the shipyards and allied threat are taken out, most enemy units will be landlocked. Defend against their air units and take out their base at your leisure.
Superweapons and Final Squadron/Balloon bombs is enough to take out the inner soviet base(and its airbases) without having to go thru the outer base first.