Fire Emblem: Shadow Dragon/Chapter 1

Marth Embarks "Marth, prince of Altea, was lucky. Though Doluna took from him his kingdom and all but a handful of knights, they could not take his life. Marth lived, and made it to the eastern isle of Talys. Talys was a small border kingdom, with no grand order of knights to boast of, but its king selflessly gave Marth use of the isle's eastern fortress. And as he saw the boy shake with anger and grief, he also gave wise counsel: "''Prince Marth," he said, "I do not doubt you love your sister something great. But you must be patient...time is on your side. If you stay here, and grow stronger, a time will come when you can help her." So Marth stayed in Talys, protected by its kind people, and the years went by...''"

Begin
The battle starts thereafter. First, surround the thief to the south. Then, use Marth to go into the village and get 10000 gold. Proceed cautiously when moving to the west, there are a healthy number of pirates ready to attack you.

The healer you get in the house south of the castle is really not a good unit. You get a much better healer soon, so save his Heal for now.

Now, comes the OPTIONAL part. You can opt to abuse Gordin (or any ranged unit) to make him 300% better than he currently is, or you can just continue. In order to realize Gordin's true potential, abuse is necessary so go ahead and have a flier like Shiida buy 2 iron bows from the armory. Give them to Gordin then have Gordin sit two squares away from the boss.

Attack, you should hit 4 or 6 damage. He will heal back that amount just by sitting at his position. You can keep attacking until your Gordin gets so good that he lands a critical and kills the boss in one shot. Easy way to make this process faster is to go into Options >>> Skip Enemy Act >>> All

Every time you end your turn, it won't show you any enemy attacks/movements. Normally this isn't good, since you want to see enemy movements. However, for this, you can spam and  to speed through battles.

Another problem that might happen is that your speed, luck, etc will get so good that Gordin will start to do two attacks instead of only one. This will out-damage his heal rate. To fix this you might need to only attack every other turn just to let him heal.

The other option is to simply proceed as usual and attack with your full forces to gain experience with a number of units.