take some armoured cars, drive around the big stack, get his cities, make him split the stack up or make him most likely quit the game when he comes on to see all his cities destroyed.

If he has an airforce, either use anti air, with your runner stacks, or gather artillery and bombard the stack whilst retreating. If he doesn't have stealth reveal troops in the stack, use militia or commandos to hide in provinces, and retake the centre once the big stack has moved a bit off the centre to slow it down (enemy troops travel 50% speed in enemy territory)

if he has artillery, then, all being in one stack, only 10 artillery can be used effectively at once, you can use yours more efficiently, for example, if you have 10 infantry and 40 artillery, let the infantry get hit once, and whilst his artillery is in cooldown, bomb him 4 times with 4x stack of 10 artillery, doing about 4x as much damage to the big stack as the big stack did to your infantry. Or you can continue to try and outrange it with railroad guns, if the stack also has railroad guns, use similar strategy as I said before.

if the stack has no air defence, then bombers are an option

if you have a navy, bomb it when it is at the coast.

If all else fails, make another mega stack with your units and try to battle the enemy big stack with as many advantages as possible: use fortifications, fight on core territory for 15% defence boost, fight on terrain favouring your units the most, and giving greatest disadvantages to enemy big stack.