Fire Emblem: The Sacred Stones/Chapter 13: Fluorspar's Oath

Beware of Selena's Bolting attack, since it can cause a lot of damage. For example, it can defeat Seth in three shots. However, if you send him in with a Sage then you should be able to get in easily. A good person to upgrade to sage is Lute, since she has high res. Only send in Seth and Lute if you upgraded her to sage, anyone else will die quickly from the bolting attack.

There are corners of the map where bolting doesn't reach, and don't worry about Selena moving to hit you. Like most bosses, she won't move. The far right town is pretty hard to save,but if you send a flying unit down fast enough you should be able to. The other town is easy enough to save. There is a pirate near it, but he doesn't seem interested in destroying the town. Have Seth visit the town to save it and move on towards Selena, healing him whenever she attacks. Also make sure Lute has some elixirs and vulnerary just in case she gets hit.