Artillery. Lots of it. Upgraded as high as you can.

And depending on your activity level (can you shoot and scoot for hours?) maybe some AT guns to stack with your infantry. The AT gun reinforced infantry can hold them off while your artillery do their work.

Attack bombers are good, but more fragile, especially if the tanks are stacked with other units. Bombers wear out. Artillery just keep shooting all day.

Yea, and RRG are always good options too.

As ZZ said, pay attention to their support units. If they're naked mediums, go crazy with attack bombers. If they're supported by AA or ints, use artillery/RRG. Always protect your own units too, so keep your own AA or ints available if they're using planes too.

And don't forget to use your terrain advantage!
Try to hit him when he's on rough terrain (not plains) and try to keep your arty in hills and mountains.
If you can get him to chase your arty through mountains that you control, that is best.
He won't be able to catch you, and your arty will do +75% extra damage.

70%

In addition to this, a ton of infantry and AT will not hurt. Medium tanks are best suited against heavy armour and lights.
Though for maximum production efficiency (in time) I suggest you to build Tank destroyers, Infantry, Artillery, (RRGs are a bonus but too expensive), and Attack bombers. You are doing good, but keep in mind that u can use the ENTIRETY of your assets, from across your nation to help in war.
Having one production type per city is efficient in terms of resource, trying to make atleast 3 unit types that can deal damage to counter your enemy's army is an even bigger bonus.

U can also use naval assistance as well as bomb his economy.

enemy artillery can immediately retaliate after you damage them using ur own artillery.
If you mean harrassing the enemy melee army by immediately retreating after shooting, its a better possibility and can easily be done in one's own territories.
I also agree with using ATs.
In short somehow being able to force the enemy to attack while defending with pan Asian units is a better deal. Maybe airforce and navy can force his hand to attack?

Yeah, that's what he means. Fire at them, then walk away for half an hour (or less to stay in range), shoot again. He can not catch you when you stay close enough to your own territory EVEN when he takes the next dot. Beware of the long roads in such places as N.Asia, and losing control of the province you're in; the speed penalty reverses 400% there (he goes from 50->100% speed while you go 100->50%)