Yeah, the game's called Total War for a reason. But the constant fighting can get tiresome.

It's less tiresome if you fight when YOU want to fight, rather than when your opponent wants to fight. So, I think the best thing to do is to go on an all-out offensive right from turn 1. The AI is called artificial intelligence for a reason, too - it cannot counter your moves skillfully. Net result: you won't have to deal with so many stacks per turn, because the AI simply won't be able to keep up with you. Net result: fewer battles.

One stratagem you might also want to consider is capturing a province or two well away from your frontline, and then giving the newly captured territory to another faction that's also fighting whoever you're at war with. Takes the pressure off a little. If it's your enemy's core province, the other faction doesn't even have to be at war with your enemy when they get the province from you. They will be soon enough, after that.