Fire Emblem: Shadow Dragon/Chapter 13

The Wooden Cavalry "The Akaneian heroes hurried in Altea, but they would not go unchallenged. Waiting for them at Fort Menedy were Grust's artillery regiment, the dreaded "Wooden Cavalry." Missiles crashed down, rocking the earth as the enemy ballistae rumbled forward. Had Marth and the League any hope of triumphing over such firepower?"

Preparation
First of all, MAKE SURE YOU SELECT MIDIA (Paladin) as one of your characters. She is necessary to recruit a character in this chapter.

Begin
These are main things to do here (in no particular order and some are described more in detail later).

A) Recruit Astram with Midia (Make sure to cover all the flanks and keep her healed. Personally, I forgot to kill the lower three ballistas on my first run)

B) Visit the village to recruit Beck. C) Defeat the two upper units with regular attack units like Ogma and Wolf. D) Use Beck to destroy most of the ballistas in the middle. Then, move in and defeat the remaining units. E) Kill the boss, Beck can do this alone. F) Visit the houses (see above) for exact details)

Beck moves one forest square at a time, but his incredible range let's him destroy. He is sorta stuck moving horizontally along the small strip.

Leave your other units to wait and let him take out most of the ballistas. Watch out for the boss, but even he too can be destroyed by Beck.

By now, the main path should be clear enough to go ahead and recruit Astram. You need Midia to recruit him.

For the boss...

Beat the last few units, and seize the gate. Save...