After overcoming Arete's attacks, another familiar face guides us through the basement of the castle.
Before the Battle
During the cut scene before this battle, we are told to only open the blue doors, no matter what. Keep that in mind.
You will get some rewards at the end of this chapter if you can unlock all the blue doors without letting the enemies see you. The only way to do this is to bring at least one unit that knows Locktouch.
Locate and Defeat the Boss
As our friend told us before the battle, ONLY OPEN THE BLUE DOORS. If you open a red door, everyone within a large radius of that door will take damage.
Also, that person hinted that you can avoid all the enemies here. If you do, then after you defeat the boss, you will be rewarded with 10000G, a Master Seal, and Boots. You only need to avoid the enemies before you locate the boss. After you have found the boss, you no longer need to avoid enemies to get the rewards.
The enemies in this map move counter-clockwise. As long as your units are not standing in an enemy's range at the end of the PLAYER PHASE, you will not be seen. To avoid the enemies, follow them counter-clockwise, just outside of their ranges.
After you unlock the first blue door and go through, you can reach a treasure chest that contains Enfeeble. If you want it, then follow one of the two enemies counter-clockwise until you can safely reach the chest.
After you open the next blue door, our friend says to open the red door next time. This contradicts what that person said earlier about opening the blue doors no matter what, so be sure to open the blue door. The red door is a trap.
On the way to that final blue door, there are some treasure chests in the northwest of the map, and another near the northeast. The chests in the northwest contain a Surefire Yumi and a Flame Shuriken, and the one in the northeast contains the Lightning tome. It is more difficult to reach these chests because of the overlapping attack ranges of the two enemies in this upper area, so tread carefully.
Opening the third blue door reveals the boss unit and several other enemies. After you open that door, you no longer need to avoid enemies, so if any of your units are farther down the map, they can make their way directly to the boss now. The enemies in the boss's room are carrying droppable items.