Mercenaries is the Prologue of Fire Emblem: Path of Radiance. It is very easy and is used to teach the basic mechanics of the game to new players. You only have Ike in this chapter. Your task is to defeat Boyd, and then Greil.
Just go and attack Boyd and he'll be beaten in a couple of hits. Heal with a Vulnerary (you will lose if you do not), and then do the same thing to Greil.
- Ike (Required)
You can attack Boyd on your first turn in hard mode. This does not change strategy, though, because you should still only need to use 1 Vulnerary after beating Boyd.