Fire Emblem: Shadow Dragon/Chapter 6X

Before Chapter 6X
Beat the Prologue fast, choose Marth and two other horsemen. Give them the experience.

Since we need to have less than 15 units to access the Gaiden chapters, we will need to kill some units. Starting in the third battle of Prologue, kill off any character that you do not want. Make sure to keep one unit to be the decoy in the following battle.

Make sure that you have killed off all but 4 units by the end of Prologue. If you did so, then you will get an archer by the name of Norne at the end of Prologue IV. Here is her dialogue (since you probably didn't get her on your first run).

I don't recommend Draug, let him die next battle. As for Norne, she is a good alternative to Gordin. I would pick one out of the two to keep. Personally, since I like the try new units on my different runs, I chose Norne instead of Gordin.

Let the enemies kill the archer you didn't choose. Wrys is gotten in Chapter 1. He...isn't advised neither. Kill him.

A whole bunch of units are gotten in Chapter 2. Keep Ogma and maybe Barst. Other than that, let the units die. Recruit Castor and Darros (and then prompt- ly let them die) or just kill them for experience.

In Chapter 3, both Lena, Nabarl, and Julian are good units to keep.

At the start of Chapter 4, you should be given 4 free units since you seem to lack in units. No worries, just kill them off too!

You also get Merric in the village in Chapter 4, keep him.

In Chapter 5, you get quite a few units also. Wendell, Hardin, Vyland, Roshea are all the units you DO NOT want. Keep either Wolf or Sedgar.

In Chapter 6, Rickard is useless. I wouldn't even bother recruiting him, but he is good experience.

Before you finish Chapter 6, make sure you have less than 15 units. Time for the first Gaiden chapter!

In War's Grip
"Not long after Marth's army freed the kingdom of Aurelis, messengers arrived from a western village to plea for the prince's help. Pirates had struck the war-torn village at its weakest and abducted its children. Marth could not ignore such an injustice, and rode with his army to the rescue..."

The recruitable character for this battle is Athena. Use her if you don't like Nabarl. At the beginning of the battle...

The enemies in this battle aren't anything special, just be sure to have a healer ready at all times. The first turn, just move everyone forward. You should be attacked by about 5 enemies. Wipe them out and then you can heal yourself before proceeding.

Use an archer to lure/kill the archer on the island. Next, you can either charge the south or the north. I would go to the north personally since there is the village there.

A mage from the south approaches the village when you do so. Use an archer again to lure/kill the mage. Keep luring in as many units until you feel that there is no risk in rushing the area. I would say 3+ units is dangerous for most of our units at this point in the game (unless of course you use a high-defence unit to rush).

Visit the village when you get a chance.

The rest is a matter of taking your time and luring in units with an archer. The boss is easy too...

Seize the gate, save the game, and note that the next gaiden chapter is 12X.