Win condition: Rout the enemy
It may seem like you have the enemy outnumbered (there are only three of them after all), and for the most part you do, but you still need to be careful. Fernand is formidable, while Bekut is an absolute force – it will take several of your units to take him down.
The best approach is to place armored, durable units in the squares close to the enemy, and place the rest to the south of the map. From there, simply allow your durable units to weather blows and chip away damage on Fernand and Berkut while the rest of your army heads in.
You’ll want to take out Fernand first, and a well placed shot from a Mage, Cleric, Sage, etc. ought to do the trick (you can finish the job with an Archer or Sniper if needed). From there, take out the enemy Cavalier, regroup if you can, and stage an all-out assault against Berkut. The more units you can throw at him on a single turn (without dying, of course), the better.
Defeat Berkut to clear the map. He’ll drop a Blessed Ring when defeated.