Win condition: Rout the enemy
This is the first truly challenging map in the game. Keep an eye on the enemy with a leather shield – his defenses are nearly impenetrable by any of your melee units. Move your army northeast from the starting point, away from the shield-wielder and toward the rest of the opposing forces. As usual, take out the archers ASAP with your strongest units first, but be careful not to expose yourself to all enemy forces at once (use X to assess). Once the archers are out, start hacking away at the leader. If you'd like, you can finish him off with a weaker unit for a massive experience boost. Silque might be a good choice, using her Nosferatu.
Once all enemies are cleared, heal up with Silque and head toward the enemy holding the leather shield. If you've upgraded any of your units to archers or mages using the Mila statue in the Thieves' Shrine, simply proceed and take ranged shots at leather shield man while placing your strongest melee units on the front lines as defense only.
If you didn't upgrade anyone to Mage or Archer, there's one strategy you can take. Head to the back of the map, where there's a small fort that restores HP each turn. Place Silque in the fort, then place your best defensive unit in front of her for protection (ideally also holding an HP recovery item. Once used, this unit can trade with Silque to obtain a second HP recovery item, assuming she is holding one). With this arrangement, hit the enemy with the leather shield using Silque's Nosferatu repeatedly until he's defeated. You'll obtain his leather shield as a reward for your efforts.