Your initial location:
The first thing you should do is lure and kill the closest Ice Dragon while you send a mounted unit to kill the thief that is running west. After that, wait for the thieves to attack you, standing outside the other dragon’s range. I suggest to put in your frontline units that can’t counter or are unequipped or can’t kill the thieves, as they deal serious damage and you don’t want your units to be attacked repeatedly by them.
After that it gets a little more tricky. There are 3 dragons north of you. Lure the middle one and kill it. In the next turn either send someone who can survive the attack of the 2 remaining ones, or one attacker and one healer; Linda with the Resire tome also works. After that you can dispose of the dragons at the side halls as you wish.
Then, the boring part of the chapter. Marth has to visit the 3 towers. Move him to the middle one first (as he approaches the door it will open). If you have the 12 orb fragments, Gato will give you the Star and Light orbs. In the east tower, you’ll find Chiki, and in the west tower you’ll find the throne.