Fire Emblem: The Sacred Stones/Prologue: The Fall of Renais

Defeat all enemies
Try using Eirika for the two lesser enemies. She should be able to take them out without dying if you wait for them to attack you. After she uses a vulnerary if nessessary, have her tackle O'Neill.

Boss: O'Neill
Try having Eirika tackle O'Neill. If her HP drops below half on his counter-attack, have Seth rescue her so that she can heal and attack again.