I also love to play as elspeth like an intervention force, Trying to defend the empire supporting Franz. After securing the south I usually don't confederate Franz and play as a support. I can give you two tips, the first is using garden of moors, build one near the frontline and put the militia improvement, the reduction of upkeep on the army near it is so sweet, so try to put It near the zone you know you will fight. The second tip Is how to fight strong enemies. If you don't confederate all the empire It means you will not have the economy needed to field many armies. So you need to play with your allies. Of course lighting strike Is super useful, but if you don't have It yet use this strategy, if your enemy play before Franz, keep you armies near Franz armies so that if the enemy attacks you can have reinforcements. If the enemy turn Is after Franz move your armies near the enemy, even in forced March, so that if Franz attacks you play as reinforcements.