This is probably obvious, but when you're deciding which characters to level up, be sure to develop a variety of characters. You want to have a mix of red, blue, green, and colorless units, with both physical attackers and magic users. Without a well-balanced team, you'll have a hard time against certain types of enemies - in particular, if you don't have a good mage, the generic armored enemies on the later Story Mode maps can be difficult or even impossible to defeat. Similarly, having a strong archer or two is super-helpful on maps with flyers, who are weak against arrows. A healer is nice to have, too.
Something that's less obvious when deciding which characters to promote is that all summoned characters have IVs (Individual Values), similar to Natures in the Pokemon games. Each character will usually have a "boon" (a stat that's above average) and a "bane" (a stat that's weaker than the base stats). Some boon/bane combinations are better than others for certain characters. A high Defense stat wouldn't be particularly helpful for an archer, who should normally be kept away from melee attackers. However, having higher Attack or Speed would be advantageous. Boon/bane isn't directly visible, but
it can be calculated. You probably don't want to spend 20,000 Feathers to promote someone from 4* or 5* unless the unit has a favorable set of IVs. Note that characters who aren't summoned (the characters you get at the beginning of the game, or from the Grand Hero Battles) all have neutral IVs.
If you summon a character you already have, don't immediately send them home in exchange for Feathers. You can use the Inherit Skill feature to "donate" skills to one of your stronger units. Donor characters will disappear after transferring a skill, though, so don't donate skills if you want to keep that character. Inherit Skill is the only way characters can have skills in every slot, and it gives you a lot more control over character customization.
Always participate in the Battle Gauntlet and Battle Arena events for each season. Even if you don't earn a world-class score, these events are still an excellent opportunity to score Feathers (and now, in the case of Battle Arena mode, Orbs).
I ended up writing a lot more than I expected. Other people who have their doctorates in Fire Emblem Strategy can probably tell you much more, but these tips might help.
This is probably obvious, but when you're deciding which characters to level up, be sure to develop a variety of characters. You want to have a mix of red, blue, green, and colorless units, with both physical attackers and magic users. Without a well-balanced team, you'll have a hard time against certain types of enemies - in particular, if you don't have a good mage, the generic armored enemies on the later Story Mode maps can be difficult or even impossible to defeat. Similarly, having a strong archer or two is super-helpful on maps with flyers, who are weak against arrows. A healer is nice to have, too.
Something that's less obvious when deciding which characters to promote is that all summoned characters have IVs (Individual Values), similar to Natures in the Pokemon games. Each character will usually have a "boon" (a stat that's above average) and a "bane" (a stat that's weaker than the base stats). Some boon/bane combinations are better than others for certain characters. A high Defense stat wouldn't be particularly helpful for an archer, who should normally be kept away from melee attackers. However, having higher Attack or Speed would be advantageous. Boon/bane isn't directly visible, but [url=https://www.gamefaqs.com/boards/204447-fire-emblem-heroes/75240906]it can be calculated[/url]. You probably don't want to spend 20,000 Feathers to promote someone from 4* or 5* unless the unit has a favorable set of IVs. Note that characters who aren't summoned (the characters you get at the beginning of the game, or from the Grand Hero Battles) all have neutral IVs.
If you summon a character you already have, don't immediately send them home in exchange for Feathers. You can use the Inherit Skill feature to "donate" skills to one of your stronger units. Donor characters will disappear after transferring a skill, though, so don't donate skills if you want to keep that character. Inherit Skill is the only way characters can have skills in every slot, and it gives you a lot more control over character customization.
Always participate in the Battle Gauntlet and Battle Arena events for each season. Even if you don't earn a world-class score, these events are still an excellent opportunity to score Feathers (and now, in the case of Battle Arena mode, Orbs).
I ended up writing a lot more than I expected. Other people who have their doctorates in Fire Emblem Strategy can probably tell you much more, but these tips might help.