The Hoshidan royal family is trapped and your mission is to free them by defeating all bosses within 20 turns. There are no dragon veins in this map but there are two chests to the NW (Master Seal) and NE (Energy Drop). Zola is carrying a huge amount of money (10000g) as well. Since your forces are divided to two, you can do this two ways: regroup and move as a cluster to easily mow down your enemies or divide your forces strategically and advance carefully, slowly whittling down the enemy forces.
The southern portion of the building is mostly composed of knights and paladins while spellcasters flock the northern portion. If your team is already composed of powerhouses, you can just divide them and eliminate all enemies in both portions of the building. Remember to save the bosses for last since they don't seem to leave their posts until you enter their attack range. More reinforcements will appear from the stairs every other turn so you'll have to move quickly. You don't have to defeat all enemies; as long as you mow down the two bosses before the limited number of turns is up, you'll complete this mission.