I'm still toying with it myself, some successes, some failures. You do have more money/loans for mercs, so a straight up battle in neutral terrain actually CAN work, but their general is going to be at least as good as yours. They do seem far more hesitant to attack in poor terrain, yet they do sometimes. Haven't figured out the rhyme or reason for that yet. FWIW, I'm having better luck in Dauphinne/Savoy then in Navara/Bearn, but that's a bit far to manage and I can't say I've been consistently successful yet.
Except for Portugal, that is. They're consistently useless. Oh, they'll happily join the war, but they don't seem inclined to attach themselves for the necessary field battles. They prefer to make a suicide beeline for some doomstack in central France by themselves. So maybe delay calling them in; they'll attach for some sieges, though, then you can use them for some mop up operations (while your doomstack tries to recover from the battle that nearly killed it). Overall, it feels easier, but maybe a bit less formulaic (or I'm still figuring out the forumulas). But you need to keep a sharp eye out for a late join by Hungary or Genoa, or whoever France cons into fighting its battles for it.
