My Problem with Corrin is that she's a debuff unit in a game where it's easy to just kill things. Her attack is really low, and it that's the hardest thing to make up for from what I see. I think instead of swift sparrow, It'd better to use Fury/Death Blow/Triangle Adept. I could see her using Vantage decently. Only thing I know is bonfire is a must.
You're definitely not wrong that debuffs occupy a weird place in FEH since it's pretty easy to just one-shot things, but I think there is a niche for bulky characters. Especially with triangle adept teams. Triangle Adept might be the best use for her, as the blue part of a triple adept team. There really shouldn't be any red character in the game, even boosted cavalry characters or all but the most stacked Raorblade users, who can kill her at that point, so she can take down her targets while debuffing for the other adept characters.
I actually use Ignis on her, since she's generally either bulky and fast enough to charge it or she's in a matchup bad enough that she's not going to be able to charge anything regardless.
Vantage might be interesting, but Seal Res is actually pretty solid on her since she's fast enough to double most characters, which makes it an effective +14 damage and lets her secure a ton of kills despite her bad attack.
Her stats almost make me wonder if Lightning Breath would be good on her, but I think her low attack and the fact that Dark Breath is the main thing that's useful about her makes that ultimately unwise. Maybe if an attack equivalent of Steady Stance and/or Hone Dragons ever come out.
