Here is my strategy:

• I play on easiest difficulty simply because it is fastest and leaves less room for errors
• I pick the French as they are the best situated on the campaign map, they have direct access to 5 capital territories from the start.
• I start off by checking the cards available. I am particularly looking for 4 specific cards: Treachery, Sabotage, Mercenaries and Logistics. Treachery allows me to take over a province without fighting in the battle map and allows me to creep up to other nations capitals. Sabotage destroys the defenses in a province which is necessary because to use Treachery the target province needs to have 4 or less defence. Mercenaries gives me more starting troops and Logistics makes my guys faster as well as allowing them to ignore attrition. Cards update whenever you buy a new card or when the world advances to a new age. I try to check on both occasions and buy/skip as needed.
• Then I upgrade my defenses to 8 as it makes sure that AI accepts bribes.
• I wait until the last 4-5 turns to see if any of my neighbours conquer the world and attack their capital. This is the core of my run however its very much RNG. The optimal scenario would be that Spain conquers every single province other than my capital and I attack theirs (this would take about 3 minutes if done properly) but from my experience of a couple of hundred resets, this is nearly impossible. The second best option is either Spain/Italy or Spain/Germany remaining with me having 2 Sabotage and a Treachery card which is what happened in this run. Unfortunately, this way you have to fight 3 battles as the remaining nation will attack you and bribing them is not possible because that means you can't attack them back.

This is a very luck based run, I doubt I will try to beat this time unless someone else starts running the game.