From: Iowan in MD/DC
I've done this twice. Once against a human, once against AI. What you need to do is close all hex sides. Use some Armored Car units to suicide shock across the river, but when you do so make damn sure that all of your other units are in Reserve mode or you'll be handed a nasty surprise in the form of several destroyed artillery units due to poor shock attack odds. The poor armored cars will just have to die for the greater good.
What you need to do then is fly hundreds of thousands of bombing sorties. Make sure you don't get CAP trapped by keeping Recon up on the place, and bombing the airfield periodically. This is at the beginning... Start your bombing of TROOPS (yes, TROOPS) to disrupt them, fatigue them, demoralize them, and possibly burn supply if they shoot their AA at you. Slowly decrease your bombing altitude until you are within the AA ceiling (you can also just look it up in the database), to be sure that they fire at your. The goal here is to burn up their organic supplies by forcing them to shoot at your planes - those bullets are then not used to shoot your troops. You'll lose planes, but not that many.
At the same time, you need to be amassing your own mega-army. This is no small task. You will need at least 10,000 AV there, and just about every tank you can get your hands on. If you're playing Scen 2 or its derivatives, send the Guards Tank Division. Actually, you should send every combat unit you can spare. Every single one will be 90+ Exp by the time you take Chungking, which is worth it later while defending. Then, start attacking. Don't be frightened - your first attacks will be very bloody, for you and more importantly for him. What you need to be focusing on is the drop in forts. Once forts are lower, your bombing attacks on troops will begin to cause hundreds (or thousands) of casualties. His combat power will begin dropping even faster. Make sure you cycle your divisions out, and split them into 1/3's during the recovery phase (recombining and re-splitting often). Always attack with as much of your army as possible (I only ever left 1-2 divisions out of any attack).
Once forts are lower, you will also need to bomb the airfield more often. This seems counterintuitive, but you don't want a single increase in forts here from no bombing of the airfield. What I did was to use a 31-plane unit of Kates (the Tateyama unit?) to bomb the airfield daily while my IJAAF 2E bombers hit the troops.
Don't be too overzealous in cleaning up his surrounded troops - once you kill them, they will respawn in 30 days at Chungking. It's better to let them wither, and kill them later. Leave just enough of your own in place to keep them bottled up, and send the rest to the capital.
Lastly, your supplies in Tokyo look low, Marshall-san .