help w/ -atk/+spd nin

Well if she will be surrounded by other fliers then a flier buff like fortify fliers would be a good c skill but if not I think just the original c skill she has works just fine. For the seal I would say guidance or flier guidance depending on the team she is on but maybe putting another drive skill on her could work too. For upgrading her weapon either +Res or +Spd. Though I don't think she would be good for fighting so heavy blade and aether I don't think are good skills. Windsweep I guess is fine but maybe water sweep or maybe one of those dancing skills like geyser dance(Just for example) could work as it increases that stats of anyone she dances for, or just use chill atk 3 since she already has that skill for now. The a slot well for more budget wise maybe darting blow could be good as to make her speed better or maybe go res +3 or something like that. Put either moonbow or glimmer for her special. Just keep dance though since dancers are very useful and that is the role she was made for after all. :)
I hope this helped, sorry if it wasn't any help.

What the heck you literally sacrificed an ike and alm for nothing: you're never going to be able to activate heavy blade she has like the lowest attack ever. You're also rarely going to attack with ninian due to her killing nothing, especially now that you've prevented her from doubling anyone with windsweep, and long cooldown specials like aether aren't good either (except for arena points). And really you replaced dance...come on. Also there's no reason for you to put iote's shield on there: she should never be tanking anything except mages and who uses excalibur anyway. Build dancers like dancers, stop pretending like you can build them like player phase offensive units.

I would suggest something more traditional: keep dance, use moonbow/glimmer as low cooldown specials or keep aether for the arena points if you're going to need a dancer in the arena, fury is usually the best choice for a slot, wings of mercy or keep chill attack for b slot, some type of support skill depending on the team you are using: if flier team, use hone/fortify fliers, if not, use guidance or keep her drive spd.