If for some weird reason you want to preserve the Bonapartist regime in France, surely your best way is to kill Napoleon, say in the Winter of 1811/12. That way, the Rhine Frontier can be defended with the hundreds of thousands of veterans whom he threw away in Russia, instead of by teenage "Marie-Louises", while if the new government has the minimal intelligence to pull out of Spain, the men who OTL were lost there are also available for something that matters. So even if all the Powers gang up on France, they can be seen off without too much trouble.