I know he's comparing it to a tank, so the back armor being weaker might make sense there, although the belly/bottom would have been the safer/more logical choice if going by armor. However, that doesn't really matter. With 9000 mana put into a single armor-piercing attack, he probably would've...
@Arsoona
To be fair, the king was right. Her idea was idealistic and a demon going back and forth to a human adventurer usually means that they are conspiring with each other.
To the readers, yes, she is the only wise one, but from the character's perspective, it's the other way around.