So there's four units in question; Dorcas (+Res -Hp), Amelia (+Hp -Res), Fae, and Fallen Robin (+Atk -Spd). I'll start with the first decision.
Should I sacrifice Amelia to give Armor March 3 to my Fallen Robin? My Amelia has her base kit, Ignis, Renewal 3, and the axe refinement. So I'd lose a medium amount of investment, but then again, I want Armor March Fallen Robin to be a partner for my Zelgius.
Second question. My Dorcas has Aether, Warding Breath, and QR-3. Should I sacrifice my built Dorcas to build a Fae? I could give the Fae QR-3 and Aether together, or I could pass on just Warding Breath. On the other hand, I could just wait and try to build the Fae from scratch, and keep my Dorcas (who's currently one of my strongest heroes). I want Fae for my new Arena Offense team.
SUMMARY:
1. Should I sacrifice a built Amelia for Armor March Fallen Robin?
2. Should I sacrifice a built Dorcas to build a Fae at some point? If I should, do I give the Fae Dorcas's Warding Breath, or his QR-3 and Aether?
If it was me I would consider the amelia sacrifice specially if the rest of your armor team is ready to go
I would not sacrifice the dorcas, you already of 3 five stars into him and he is arguably the best green axe infantry, were amelia is definitely not the best green armor
Warding Breath is a very rare skill, inherit that instead of Aether. And Aether is useless on dragons unless your running Warding/steady breath on them. Given that dragons are absolutely the meta rn i would reccomend the sacrifice.
Only give armor march on units that you plan to go duel phase on, because they get more use out of the extra mobility. Things like Armor units running Bold Fighter are much better users of armor march. Armor mages with blade tomes also enjoy the skill as well. In short, wait till you get a better candidate for armor march
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes