C slot to H!Kagero (+spd -atk)

maybe attack smoke or a buff. But why not keep even attack wave? It fixes her -atk every other turn.

Don't give her CC, especially with -atk. You probably want her to focus on player phase since there are probably other armored units more deserving of the QR3 seal. Also, even atk wave is fine.
My recommendation:
Death Blow 3.
On even turns, even with the attack bane, she'll be hitting at 59 attack with a guaranteed double. I'd also suggest considering glimmer for her special, since it would be guaranteed to proc on her second attack each time she initiates, whether or not she gets countered.