Here's a possible combo to try to weaken your enemy.

Build some inquisitors. Send them into neighboring provinces. Raise the zeal to very high levels. Get it as close to 100% as possible.

Next, launch a bunch of spies into those provinces. Stir up trouble. Force your opponent to flood the territory with units in order to keep them under control.

Next, launch a crusade. Send it into high-zeal enemy provinces. Snatch all their troops. Leave their border territories weak and defenseless.

Boom. Once you leave, the province rebels. You've snatched most of your enemy's units (which, admittedly, they're no longer paying for, since they're in the crusade... but they're unavailable until the crusade is over). With luck, you can grab the territory without having to go to war.

Fun fun!