Fire Emblem: Shadow Dragon/Chapter 19

Manakete Princess "The Sable Order fought valiantly to keep Marth's army from reaching Chiasmir, but in the end Grust's knights buckled: the world's mightiest only in song. The League marched on to the Fane of Raman, a holy sanctuary built by the divine dragoons to house their most priceless treasures. Here another story was set to begin..."

Preparation
Before the battle, use the Master Seal obtained last battle on a unit that is level 15-20 (the normal recommended level to promote). Be sure to also bring a thief along. When ready, just start the battle. Also, set aside one spot for Bantu, he is needed to recruit, interestingly, the boss of this chapter.

Begin
You might have noticed right off the bat that this map consists of a whole bunched of locked rooms...and seemingly few enemies around them. That is partially because most units are at the top AND that some enemies are inside the rooms.

There are 10 doors.

9 10

1 2 3 4 5 6 7 8

With 1 being the room in the upper-left corner and 8 being in the lower-right corner. 9 and 10 are the rooms to the extreme north. Here are what they contain:

Notice that the rooms alternate from left to right, anytime you are stuck this is a sure way to know which room has treasure and what room has enemies. Along with the Master Seal you get in one chest, you get another from the enemy sniper behind Door 2. By now, you should be sticking with a team of 15 or so strong units. As normal, no need to rush using the Master Seals. Wait until next battle's preparation.

Anyhow, lure/kill the mages and healers. Then, talk to the boss with Bantu. The conversation is as follows:

Seize the gate.