Fire Emblem: Shadow Dragon/Chapter 18

The Sable Order "For Gotoh to create the magic that could defeat Gharnef, he needed two orbs: the Lightsphere and the Starsphere. Marth and company set off to Chiasmir and the Fane of Raman to find them, but in their path stood a cohort of Grustian knights known as the Sable Order. The bridges across the strait would soon be choked with the brave and the dead..."

Preparation
Faster moving units do well on this map. Also, Marth's rapier and Shiida's Wing Spear can do hefty damage. Only drawback is that there are enemy archers. Ridersbane is also good.

Use the Master Seal from last map and then start the game.

Begin
To begin this map, just move your units forward. Keep your units that have the special weapons (ie. Ridersbane) in the front. There are 4-5 cavaliers that immediately rush from the south, defeat them first.

By the time this happens, the group in the middle should have been provoked. Kill them with units like Marth and Shiida. At the end of turn 5, a pegasus knight, Est, will appear to the left. Marth is needed to recruit Est. Est's stat growth is weak compared to other Pegasus Knights.

If you aren't going to use Est, give her Ridersbane to a cavalier or maybe Shiida. On the middle island, there is an armory, shops, and an arena. Unless you really need the weapons/items, don't bother for now. You can always go back before seizing the gate to buy items.

Reinforcements from the southwest fort should be heading toward you. Lure them into the middle section and then fight. You don't want to risk losing a unit to the other enemies in the southeast.

On turn 14, a paladin will spawn in the fort in the middle. Use one of your rear units to get rid of him. The reinforcements in the lower southwest corner will stop spawning at the end of turn 15. The paladin will keep spawning though, every other turn until 18.

Feel free to use the arena in the middle part of the map to level up any of your lower-level units.

Charge the lower part of the map and let the few units come and attack you. The reason for this is that one of the units near the castle has a ridersbane and can pose a threat to one of your units.

If you triggered the two horsemen to attack you, hope that all your cavaliers that can be attacked are at full or close to full HP. If you didn't, you can choose the luxury of picking them off with Beck. Or you could use Shiida to go around and kill them.

Seize the gate and continue to chapter 19.