Myrrrrrrrrrh

Nope, her Atk becomes way too low at -Atk.

There is no such thing as "this character becomes unusable with x IV", any character can be made good, even more if it's a top tier unit. The +res makes her mixed bulk very hard to deal with and she is still able to kill reliably with QR even with -atk.
Her worst bane would be -def, but it would be workable even in that case

I'm sure it will be fine. My first one was -Attack +Defense and worked fine until i got one with -Hp +Defense. It might be different if you were trying to +10 her, which I'm assuming wasn't what you were getting at.

Her dragonstone gives her +3 atk and is 16 might, it's not the best nature but she'll be fine, plus she has adaptive damage. +res makes her even more tanky.