Big stacks

- Take back the provinces it takes as soon as he moves on.

- If it is moving, it can't fire artillery. Yours can.

- If it stops, use rockets.

flank it,recapture every thing behind it

if/when it it splits, is the time to attack

You can also sometimes lay a trap for it

The players that use mega stacks like that tend to send them on a multi day excursion.

So patrol over it with a plane for a few minutes just before it captures a province, and then leave it in the dark. This will show the planned route at that time.

So with any luck you can muster some forces 12 to 24 hours ahead of it and build a fort to protect your forces. Every unit has a vulnerable terrain of course, so if it is heavy on armored units lay your trap using infantry types in the mountains or a city. If it is heavy on infantry types lay your trap using armored units in the plains with a fort.

As mentioned above: Rockets for when you do throw up this "spike mat" and bring it to a halt. Once you have it pinned in place it will not escape your rockets, and your rockets will not hurt your own forces. By rockets I mean level 2 and up of course.