Training Begins is Prologue III of Fire Emblem: New Mystery of the Emblem.
Keep in mind that after completing the battle, Jagen will present you with a Yes or No question; your answer changing not only the boss of the next Chapter, but also a future recruitable character. The general consensus is that answering "No" leads to an easier fight in Prologue IV, but "Yes" leads to the recruitment of a more useful character.
Subjective: The following part of this article is subjective to the editor's writing. Therefore, it may not work for everyone.
The player should build a formation to protect Ryan and use the lower fortress to increase the survivability of one of the more fragile units.