there's two simple strategies when it comes to France in ironman Europe, depending on who you are playing of course
Ally France for the entire game to kill Austria and/or Castille
Kill France early with Austria and/or Castille
if you don't use any of these options....France will find you, and France will kill you
I don't think is really true.
I'm currently playing an ironman Friesland>Netherlands game, and initially I allied with France to take apart Burgundy. I of course did not really give them any land during those wars though.
However, once Burgundy was destroyed, I then switched sides to Castille and started joining wars against France. We have won every single war we fought against them. Admittedly, our efforts to dismantle them haven't been great, as Castille or Austria (who hates me unfortunately) is almost always the war leader and never releases that many nations.
Despite this, it's 1600 and I'm really not that scared of them anymore even though they still have almost all of modern day France under their control. Yes, they still have 80,000 manpower and a force limit of 65, but at this point I have a force limit of 55 and 60,000 manpower (from building every manpower building I can), and more importantly, I have loads of money and can just use mercs so I don't lose manpower. I'm also 2 military techs ahead of them, and I have better morale than they do (defensive ideas + reformed religion bonus = +30% morale).
Honestly, I think France is weaker in this patch than they used to be.