I always made the error to take offensive ideas as burgundy and try to win france, having this problem mainly, the fact that if i won a battle they would retreat to narnia then come back in flanders.
Lemme say eu3 mechanics are too easy and human balanced, you know that if you win a battle then you'll win the war, instead in eu4 this won't happen and is more challenging. Anyways i think that the solution to win AI is just use his tactics. I started a game with burgundy wanting to become netherlands (actually very difficult because you're outside HRE), and just wanted to play friendly with france just because the decision will give automatically the french region to them, so there was no intention in fightin to defend some provinces i would already have given to them. Ok said that, the funniest thing is that france dow me 3 times, the first one they mega-stomped me and i managed to lose just picardie and calais, the second and third one i mega-stomped them. They even declared war on me, i made peace on day one releasing vassals because i already was in war with austria and castille, and got re-declared war from france the day after. I wonder if the ai gets the -5 stability hit, and why the hell they wanted to accept peace the day before then, re-dow day after.
Anyways to keep things on topic i won the last war having less armies than france and ofc less manpower, just using their stupid tactics. Defend always, keep main army full of men and when you win a battle in your territory there's a high chance that the enemy will retreat in the nearest province, then annihilate the army. If they gonna retreat to narnia just salute them and wait for next round, until you will check that their mp is off and almost the whole army shaken, know your enemy and for example about france, they usually retreat on provence or ile de france, sometimes i've seen them on armagnac or roussillon if they have. Against burgundy i've seen them a lot of times regroup in provence. Go there straight away without changing direction everytime the enemy enters a new provinces, or just watch the white flag under the army. If things go well you win the war.
So to sum it up if you want an higher chance to annihilate the enemy's army, look at your history and think at all the times you've lost your army. 90% of times for me were on enemy's territory, so i just reversed my tecnique, and played defensive with def ideas.. That +0.50 morale is really good.
In general i like this feature as i think that in eu3 it was too unbalanced, unrealistic, and easy in that sense. Here is just a challenge, as it should be.