They call it the bomb for a reason! NUKE IT

Lead their forces deeper into your territory. They will have to leave troops behind to deal with rebellions eventually.

If they were to travel by ocean, they wouldn't probably have it defended and since that stacker wouldn't use an aircraft carrier due to the sheer size of his army, and they would attack anyways. Use a submarine but in all cases a bomb is needed.

The only viable option here is to wait for the nuke rockets to be researched and produced and then you can nuke it.

its easy to kill them, lol. Doesnt need nuke to kill it. If the game has max stack, commitern doctrine will useless, because commitern doctrine need big stack. if stack gas maximum, i just said just delete commitern doctrine because they hp lower than other with normal standard. Its like you give commitern doctrine a punishment.

It really isn't easy to kill those huge stacks. Any player who makes massive stacks like that has the common sense to add arty, aa, and at to it. You couldn't throw anything at it that wouldn't be easily destroyed, unless you made a mega-stack as well.

if these huge "stacks" are anywhere near the coast your best bet would be to bomb them with battleships, if they are planes put SPAA or cruisers, fighter planes or rocket fighters are also always an option. i have faced a stack like this before. use paratroopers to attack then use planes to bombard it (if possible) then get artillery to attack from all sides because they'd have to split their artillery to attack all of yours, then they become vaunerable to all attack. after that if its not dead, just send 10 stacks of units. (if you even have access to all these units)

I said its easy because my strategy is always use arte. But if your strategy only use closed combat, its hard. Idk what his army stack, but in that position you can use Battleship to bombard it. If its stack use normal arte and aa. Use stack 10 arte (full upgrade) + 3 rocket arte (min lvl 2). And if he still near sea, bombard from battleship too. If you have rss, build rail road gun.
If he didnt have any bomber, use Contra strategy. Build armored car as much as you can. Capture his empty province. Do hit and run to take control his city not for capture it. Just for destroye his building, so it will make him slowly to produce army.
If he has bomber, use your head to destroy his airship.
Doesnt need mega stack to defeat it.

Agree with HyM, sure you can stress over the max stack, nuke em if you got them, but really, when i face a max stack i just grab a bunch of fast units, 3 or 4 of them follow the max stack around recapping any areas he leaves. The rest run about capping any of his vacant territories. The maxer will either be forced to break up his stack to stop your units, or lose production and morale until he has only the territory the max stack is on. In the case of a flight of planes, bomb the base they are coming from, or better yet cap it. (if you click on the enemy planes when you can see their details it will also show a line back to where they launched from. Kill the airports he uses until the planes are either out of range or out of airports. Planes stuck on the ground are easy kills.
One caveat, if they have max stacks in multiple areas AND his territory is still defended, you got a coiner, unlesss you want to spend a lot of gold, don't waste your time trying, start a new game.
Just my humble opinion.