Fire Emblem: Shadow Dragon/Chapter 21

Clash in Medon "Grust would harry the League no more. Marth led his forces on to Medon. However, Medon's Dragoons were ready to waylay them at the border; countless reinforcements hovered within the fortresses, waiting to strike...Would Marth be able to lead the League safely through the gauntlet?"

Preparation
There isn't much to do on this map, just that picking fewer units will make it easier to defend them. Start the map when ready.

Begin
Basically if you don't have any weak units and all your units can fight for themselves, you are good to go on this map. The reinforcements appear in sheer numbers, making it impossible for your good units to kill them all in one turn. This makes it hard to defend a weak healer or a mage, which is why I recommend taking less units into this battle.

You can defeat the Generals at the beginning without provoking the mages near the castle. At about turn 6, about 6 enemy dracoknights/pegasus knights will appear. It takes about two turns to catch and defeat all of them. Of course, have a healer run around with Mend ready to heal.

Reinforcements will spawn in about every three turns. They only come in three times total the whole battle. If you have a REALLY hard time, move all your units to the right side behind and in the forest. When the reinforcements come in, eliminate them with Beck and archers. Then, you can turn around and defeat the reinforcements that spawned on the left side.

Of course, after the reinforcements end for the third time, just go attack the generals and finish the map like normal. After this, kill the healers and then move onto the boss.

Seize the gate after this. Save the game.