There will be 3 Pegasus Knights coming towards you, which shouldn’t be much of a problem.
There will be 3 Social Knights, a horseman and a thief coming from the north. Kill them with your stronger units.
Visit the villages before the thief destroys them. Be careful with the mercenaries. Head north.
The fire dragon is a dangerous enemy. Use the Dragon Killer sword, ice magic and Linda’s Aura spell. Be careful with the sniper, too. In order to lure the dragon without luring the Sniper, have someone stand in the space shown in the picture below.
Now just hurry and kill the ballista standing at the castle gate and warp Marth. If you take too long, reinforcements will come out from the forts.