The girl hero is the one that cast the ice wall, the clash between the demon fire and the holy strike caused an explosion which damaged the surroundings and the heroes.
The boy hero lost the exchange because he wasn't quick enough in fully casting the holy strike.
Where did you come to the conclusion that it was to harm Ferrius? Myard was supported by Ferrius, the manga even clearly states that it was the combined forced of Myard and Ferrius that trampled the Canoa country (chapter 1, page 32).
And its only logical why they did it. Canoa lost the war, the...
The reason Highserk postponed the invasion was because they had to rebuild the annexed territory that suffered from the scorched earth tactics.
The timeline goes as followed:
5 years ago, Mayard and Felius convince Canoa to attack Highserk, which they thought was weak from the constant war...