I'm not sure what you mean by final battle X... I'm assuming you are referring to dark night X the 8th map.. If that is the case the silver Valkyrie wasn't used at all. It was Platinum Valkyrie Thetis. Either way trying to replace one class with another if you are using those videos may not always work. Specific class do specific things.
Trying to replace a Valkyrie with a non-valkyrie will result in two things. That unit will not have any magic resistance(most likely), and you'll be down in UP as Valkyries give you one each unit they kill.