Is "Sow Dissent" the trick to being able to attack one county from a large kingdom without having all of the kingdom's troops stomp you or there another way? And where exactly to put the Chancellor to sow dissent... in the county I'm gonna attack, their duke's county, or their king's county.?
My dink little country next to France would like to use a casus belli on a county, but it seems essentially untouchable. How do yall do it?
No, there is no easy trick you can pull to break counties away from France.
If you're lucky, you can have them try to declare independence, after relations with the king get bad. But they would only be "easy picks" for you, in the case that they win their independence struggle. If you attack while the independence war is ongoing, the rebel count will likely sign peace with his king as soon as he realizes you are about to take his land away.
In general, there is no easy trick to get a grip on larger kingdoms. If there was, it would be an instant "win" button.
Just stay watchful, keep an eye on the kingdom, and hope that they end up in some sort of internal struggle eventually. Or better yet, watch the wider world as well, because France might just have a run of five excellent kings, and you wouldn't get an opening even if you waited a century.
You can do like the Anjou dynasty that ruled parts of France: They never became kings of France, but they managed to seize a whole bunch of other places:
conquered Naples, ran the place 1266–1435
conquered Sicily, ran the place 1266–1282
tried to seize the byzantine Empire, didn't work out though
usurped the (landless) crown of Jerusalem 1266–1435
conquered/seized Hungary through a claim inherited by a woman, 1301–1414
inherited Poland and ran the place for a short while, 1370–1386/1399
I bet they too would have liked to run France, but as they never got an opening, they put their sights elsewhere
