Fire Emblem: The Sacred Stones/Chapter 11: Creeping Darkness

Defeat all enemies
Welcome back monsters! As you already know, monsters are pretty weak compared to the human enemies you fought but they make up for it with numbers and ambushes. Recommended units would be a thief and a ranged attacker with an antitoxin; preferably a bow user.

Have two solid units and your ranged attacker down through the narrow left part of the map, straight toward the boss. The ranged unit is for the bonewalker in the building that has a poison bow that will try to poison your other units. The other two monster next to the treasure chest have poisonous weapons so be alert with that. Head down with your small group toward the boss and afterward, head to the right to meet up with L'Arachel. Have Eirika talk to her and once she's recruited, have her talk to Dozla.

While the small group is taking care of the boss, the rest of your team, along with Colm, should move to the right inside the building. Take care of the monsters inside the building while Colm opens the chests on the way. Some monsters will come in from the entrance on the right to attack you so be careful. Once you get to the door on the bottom of the building, one monster will open it and a group will swarm in and ambush you. Just stand your ground and you'll be fine. Try not to kill all of the monsters until you open all of the treasures.

Boss: Wight
The wight is a stronger version of a bonewalker, but it'll act the same as one. It only has a Steel Sword so it won't hurt you that much. It's not going to put up much of a fight so have a weaker unit take the experience since it gives more than the monsters around it. In a nutshell, he's not too different from other monsters.