Well what can I say
Think outside the Box
Fight the Air Units from the Ground
I thought I had a Replay saved somewhere but I could not find it anymore
So I just loaded an older Savegame and made a quick run through the scenario to get a replay file
As it was some time ago I last played Soviet Corps it's pretty awkward and I blundered several times on the Eastern Island with the Airport and butchered several Infantry Units from my troops. Same with my southern squad, impatience and a sloppy approach cost me some, but it's the last scenario of the Campaign anyway and you have plenty of Prestige and Units to burn so it doesn't matter. But since you have pretty much time to conquer all the other victory or non victory hexes even holding back to get the last non victory hex city you will get the idea anyway. I think with a little bit of discipline and patience you can clean out the whole map easily, even without a disband. At least on Colonel or up to FieldMarshall Level.
So I deployed NO Air unit (except a Para, can be be any Unit to conquer a Hex as well, the Idea with the Para is to evacuate it from the Island later on. I messed this idea up with my incompetence as you will see

)
All my best towed AA Guns (these are your key to victory)
Plenty of Arty (note while I messed up a bit in the south with towed Arty, it's better to use Switch Units doesn't matter if it is Arty or At or even KV2 with a switch, Range heroes are welcome

)
Never mind a few heavy Blows from the Japanese Air Force, just patiently pound them with the AA guns
Also soften Ground Units with your Arty and finish them off with the few Tanks you have on the Battlefield.
Make a slow and safe progress step by step. Don't run out on Ammo especially with your Arty
Just watch the Replay and you will get the Basic Idea, just sorry I could not give you one with a little bit more accuracy and sharper gameplay (after all this a just a hush hush fight using the old tactics again)
edit
so I took a bit more time to play it once more and with some Responsibility and here you have a Decisive Victory for the Second Replay
btw I posted it on some other thread
Going Groundbound (without any Air Units in all Scenarios) is nice alternate Task to go through Soviet Corps