I've actually thought about this before, but Fire Emblem, to me at least, doesn't have many characters well suited for the Death Battle format, which takes tangible feats into account and uses real world mathematical application to determine outcomes. This mainly stems from the fact that the vast majority of characters, despite the multitude that exist, are badass normals with little to no confirmed feats. Even the main lords who've killed dragons, gods, or dragon gods mainly did so because their weapons were specifically made to kill that exact being, and not necessarily anything else. Up until relatively recently the games didn't have full-on cutscenes (save only the 3DS and Tellius games) so there wasn't really any visual feat confirmation in the other ten games, or at least not as many that couldn't be written off as "Sprite logic" or similar things.
And even then they don't add a whole lot. Most of the cutscenes aren't full-on fights, and some that are don't showcase the characters doing anything too crazy. Lucina in the Bad Future cutscene didn't do anything too extreme, Alm vs Rudolf was a dream, Celica was
...
And not to mention their defensive feats are non-existent, since the only cutscenes that show them being struck actually hinder their feats. Looking at you, Chrom, Elise, and Celica. This is especially true when considering me being struck during gameplay is likely just a case of gameplay and story segregation.
So with that said, I could really only see the following as more high-tier picks.
Robin and Celica - Both can utilize lightning and fire due to their magic, which are more tangible and measurable than things like holy light and razor wind, which are much more abstract and harder to calculate for. However, both are disadvantaged a bit since Robin would rely on his tomes (all of which could break, unless they specifically go for his Fates incarnation, though that only applies to Thoron...or unless they give him max Grandmaster stats, Limit Breaker, and Armsthrift, but those aren't canon) and could have them knocked away, while Celica casts from her own health, making her unencumbered, but more squishy.
Roy - Fire is a measurable thing. Might be hindered by lack of true feats by virtue of The Binding Blade's sprite based combat.
Ryoma - Lightning is a measurable thing, unlike Xander's "sword that uses the power of darkness" Siegfried. You could likely calculate his speed and strength from cutscene logic, making him doable.
Chrom and Lucina - Not many feats on their own, but both leaped into the air about three times their body height in the Dual Falchions cutscene, which has to count for something. Also Chrom broke through a stone wall during training, but that doesn't have as much context to go off of.
Corrin - By virtue of versatility due to her draconic powers. They might be able to pull from that.
I haven't played the Tellius games because they're so hard to find, but Ike also gets mentioned a lot, from what I've seen. I can't weigh in too much though.
tl;dr: Fire Emblem, in my opinion, doesn't lend itself well to Who Would Win/Death Battle types of things due to the nature of the series focusing on badass normals strategically striking foes, and most don't have super powers to back them up, or impressive feats that can be used as a basis of measurement, save for the few I mentioned.
Just my take on it.