Win condition: Rout the enemy
This shouldn't be too terrible challenging of a battle.
First take out the archer, then proceed carefully using familiar strategy of strong units followed by ranged units. Make sure your new friend Clair doesn't take too many hits – she'll dodge a decent amount of incoming attacks but not everything, and won't be as potent as a Pegasus Knight you've already leveled up a few times (if you have one).
Once threatened, the enemy will retreat to a nearby tower and restore health when their numbers dwindle – finish them off with a unit that can deal substantial damage. It's best to defeat the enemy commander early on so that he isn't the one restoring HP from the supply post at the end of the battle
Defeat the enemy commander and all enemy units to clear the map.