Help with Halloween Kagero building

This is pretty budget. You could even throw Brazen Atk/Def 3 or Close Counter in A slot with QR3 or CD3 / DD3 in seal.

You sound like this iv is horrendous, this is her best iv. It makes her base kit even more helpful as she has high Atk and comes with Atk Wave and Mirror Strike. Making her hit even harder. I think you should just stick with her base kit.

If you give her brazen def/res and aether she just might solo infernal Aversa for you. Add...close def, or something.

You could run Rogue Dagger on her. That way she self-buffs for +6 def/res whenever she attacks, and can be run with a bunch of wards to be super tanky.

That's her perfect IV set! Congratulations! Her best A-skill is Close Counter, but her default Mirror Strike works fine as a budget option. Other than that, give her Swap and QR3 seal for a mixed phase set.