After a practice battle, you are brought before the king of Nohr.
Normal and Hard Difficulty
When you first start the battle, walk onto the Dragon Vein and choose Dragon Vein to clear the debris. Only Corrin can do this.
The healing area is a 3x3 square. As long as your units stand on these tiles, they will regain HP on each turn.
Put Jakob/Felicia in the center of the square
Put Corrin on the left side and Gunter on the right. Make sure that they are still standing on Healtiles. It's best to put Corrin on the left and Gunter on the right because Corrin's weapon is effective against Rinkah's axe, while Gunter's weapon is weak against it.
Don't use Gunter more than necessary. He may be strong, but he doesn't have good stat growth, so it's better to have Corrin and Jakob/Felicia deal with the remaining enemies, and just use Gunter to ensure that the enemies don't kill Corrin or Jakob/Felicia.
Focus on defeating Rinkah and Kaze while your units are on the Healtiles. Then after those two have been defeated, kill the remaining enemy units.
In Lunatic mode, you will need to protect Jakob/Felicia from taking too much damage. Focus on getting rid of Kaze and Rinkah as soon as possible. But be sure to make Jakob/Felicia gain some levels during this chapter so that he/she can provide enough support in the next one.