The idea that a large AI nation, regardless of difficulty or settings, can't be dealt with by a similarly large player nation is a bit silly.
Drag the AI armies into view by sieging a province or two, be sure to have a scout stack on look out and be ready to move if you're genuinely worried about a defeat.
Create an army with a 10-12 regiment advantage to face them and feel free to go over force limits, obviously you'll have sorted your income before you go to war right?
Create a second stack at about half their size.
Lure the AI into an offensive battle, to get any defensive bonus you can, have the second stack move a few provinces into their territory.
Win battle, AI retreats, get to AI retreat destination with second stack before AI get there. Crush AI.
If its France you're dealing with then they have dozens of small nations that you can have them release, all of which will be an immediate drain from their power. For extra effect see if you can find provinces where the AI has invested in buildings, as releasing the nations that take those provinces should destroy the buildings.