Fire Emblem: Shadow Dragon/Chapter 20X

Before Chapter 20X
For Chapter 18, remember that Ridersbane/Poleaxes are invaluable. Est joins your army, but dispose of her. Also, I wouldn't keep Etzel from last chapter in favor of Merric.

For Chapter 19, remember to get all the chests. Get both orbs also and recruit Tiki with Bantu (it's easier this way to get Chapter 24X).

For Chapter 20, use master seals from previous chapter. Then, just rinse this map as if it were a normal run. Lorenz is pretty much useless compared to Horace. Kill him instead of recruiting (or recruit then feed him to Camus) Be sure to have only 15 units left, then to the gaiden!

Men and Monsters
"Having won the day and put Camus his Sable Order to rout, Marth and the League moved on to the mountains in eastern Grust, to crush the last of the opposition The surviving Grustian soldiers, in a mad attempt to hang on to their lives, took all the residents of a nearby village hostage to keep Marth from striking. However, they could not have picked a worse village to harass..."

Preparation
Only unit that you should really bring is a thief. Units that move farther are an asset in this battle. Also, weapons that deal with Manaketes/Cavaliers well make it a cakewalk.

Begin the battle.

Begin
Indeed Ymir, the recruitable character, is standing guard. He is on the upper right side of the map. He doesn't move, and you should talk to him with Marth when possible.

However, it is more pressing to deal with other threats first. Also, I wouldn't advise opening any chests until you pretty much cleared the map.

Split your army into two parties. One to the left and one down the middle. Both paths have a considerable amount of danger, so be sure to put good units in both parties.

The left side will engage in fighting first. The broken bridge is a perfect place to have a paladin or two.

Meanwhile, two paladins and a cavalier will be rushing the middle. Kill them and most likely, you will trigger the three relatively strong generals to fight

Defeat them the next turn, and then move a units with a large movement (like a horseman) to the right and kill the manakete. Ymir should be right there, standing guard.

Talking with Marth and...

For the left party, you should have confronted with a mass of paladins moving in from the northern part of the map. They are relatively weak, all of them only have steel swords/lances.

The manakete there should be taken out with care from a use of a Dragonpike or a Wyrmslayer.

Now, the whole map should be clear of every enemy except the boss and the two bishops near him. Before, worrying about them, let's get all the chests.

First, the island to the left (first island that your left party came upon) has a Killer Lance.

Moving onto the island that had the manakete, there's a killer bow in that.

Now, moving onto the island that Ymir was on, there are two chests there, they have a Wo Dao and a Killer Axe.

The two remaining chests are near the boss, so we can't access them just yet.

Instead, move your units and Marth to the upper-left. Use a paladin or unit with long range (Shiida works) to kill both bishops in one turn.

Then, we can access the two chests. They have a poleax and a Longbow.

Now onto the boss:

This boss is easily killed by long range attacks, but Armorslayers and such weapons work too.

Seize the throne and you will get dialogue. End of gaiden. Next one is Chapter 24X, for this one you need to kill Tiki off and not have the Falchion. More about this later.