After defeating the first wave of enemies, you continue farther into the hidden kingdom.
Defeat the Boss or Have All Units Escape Within 20 Turns
Another familiar map if you have played Conquest.
There are escape tiles in the northeast of the map. If you move a unit onto any of those tiles and choose Escape, that unit will leave the battle.
You will notice that there are many enemies in the southern half of the map, but at first there seems to be no way to get there. However, if you use the Dragon Vein to the left of where your units start, you will create Replica units in the southern half of the map that can battle the enemies down there. It is very worthwhile to do so, because almost all of the enemies in the southern half of the map will drop items when defeated.
Be aware, however, that if you make a replica of Azura, she can't Sing. Furthermore, if you are playing Phoenix mode, keep in mind that the replicas will not come back when they die.
When one of your units is healed, the Replica is healed as well, and vice versa. This means that you could simply leave all of your replicas standing on the Healtiles down below to help your northern units heal up as they battle their way east.
You should focus on sending your northern units east so that they can escape if all of your replicas die down south (or if you just don't feel like gathering all the treasure down there).
Your northern units should be able to reach the eastern escape tiles well before turn 20, so you can just have them stand there while your replicas clear away the enemies down below. The boss unit will also drop an item when defeated, so assuming that your replicas have not all died, you should attempt to kill him to end the battle.
If you have not completed either objective by turn 20, you will get a Game Over and will have to try again from your last save.