To clarify a few things here: the prince's actions make sense within the context presented.
As much as it seems that he's making a strategic faux-pas because of an over reliance on Elise, the fact is that it's not just him committing the mistake -- it's his entire general staff, too. Moreover, it's already been well established in past chapters that, in Elise's previous life, the Crimm War was a disaster for the Empire due to a number of reversals and strategic successes by the Francois Republic -- meaning, absent Elise's past knowledge and contributions, all of the previous strategies would've worked because they already did in her past life. Literally none of her contributions up to now have relied on innovative military thinking -- she's basically just been using information from her first life as Elise to recognize recurring events and dealing with them appropriately.
Moreover, it is absurd to think of Elise as a 16-year-old girl. Between all three lives, Elise has likely lived 60+ years (it is unclear at what age she died in her first life but likely over 17, she died around ~30 years old in her second, and she's about 17-18 at this point).