What to do with a +hp -atk Caeda

It's hard. She's a Jagen class unit even before an ATK bane. Her only saving grace is that she's a flier. I've been thinking about inheriting Ruby Sword over to my neutral Caeda - not sure if it will help but her primary role on my team has been mobility aid (definitely give her draw back or reposition), fortify fliers buffer, and in theory a green mage counter. The problem has been that while she can tank most green magic she usually can't take anything but glass cannons out. I thought Ruby Sword might improve that....still debating with myself.

Alright I'm gonna suggest a very weird approach.

Sure she is gonna be very bad at damage dealing the classic way. But she could be a very good mage bait, especially in a flier team.

I would give her a very weird skill for A : Water boost from Berkut.
Due to her HP IVs she'll have more Hp than most mages. Maybe put a HP seal on top of it to make sure you are safe.
She'll reach a very good 40 res with that which is enough to tank any green and red mages.
On the other hand she'll also tank blue mages not too bad.
I ran some test, even a fully buffed Blarblade Linde will not be able to ORKO her if she is running flier buffs.
Well she'll still be suceptible to those mages but not as bad as other red units.
Even if you don't use her for a flier team, having 40 Res is still gonna let her tank a lot of mages.

Against a unbuffed Aura Linde and according your Caeda is also unbuffed, she'll take 22dmgs. Just think about that.

This is like the lowest investment build I could think of to make her usefull.

Of course you could try other things, you could probably replace her weapon for a better one but it'll still work okay as an anti mage + hector counter.