I don't think this would work for multi-player, but it's great against the AI.
Get "in his face" by taking a province near an AI opponent. If this province is a "chokepoint" on the map, that's ideal! Then defend the heck out of that province. If you have decent PD, buy as much as you can afford. If you have any commanders that summon allies (maybe even freespawns!), park a few in the province. Build a laboratory and set a couple of mages to monthly castings of some sort of cheap summon.
Basically, the idea is that the AI will try a big push with all available troops to kick you out of your province. If you can resist that push, then you can grind up an insane amount of your opponent's resources. Then, each subsequent attack will be easier and easier to repel. While he's wasting troops by throwing them against your unbeatable defense, your armies can conquer other provinces to add to your strength. He's stuck at a standstill, while you're still making progress. Guaranteed win for you!
A human opponent will figure out what you're doing quickly, and the trick probably won't work. But the AI seems to fall for it nearly every time.
