Rhea lies about a lot of ****, and that makes things confusing all the time.
But from my playthrough and from what I've gathered, Sothis is the strongest being in the game, since she's pretty much the
God of the continent, as is the norm for any Fire Emblem game. I'm debating whether or not she scales to the Sword of the Creator directly, since it
is her bones and heart powering it, so it's kind of like her physically attacking in a sense? It is a bit difficult to figure out. This all comes from Claude's explanation of the Sword of the Creator being able to split a mountain or whatever.
Rhea and the other kids of Sothis come second. She's her last kid, but she's also a hecking dragon. Also she could barely live a magic nuke before eventually, from what I assume since she doesn't show up ever again after the incident, ****ing dies later on. Props to her on the feat.
The scene where Fort Merceus gets nuked is interesting. It shouldn't be as powerful as an actual Hiroshima, the Fort isn't even that big. It's probably more like an MOAB, being around Large Building to CB or MCB level.
But she didn't get totally vaporized, so that's important. Because if you take the Black Eagles route, guess who's fighting Rhea. Definitely you with your whole magic Sword of the Creator and that jazz.
Everyone else's durability isn't that great. You're human, and the game stats don't mean much to me IMO. It's like how some people treat Pokemon where they meme Ratata to be as strong as a legendary because of Facade or something like that. It's a numbers games, man, they shouldn't all scale. I'm only taking the actual cutscenes and context from game literature (yeah i ****ing read the books in the library, don't judge me)