For Nidhogg stage you need only 10 rainbow to knock it.
One for Kerria + debuff girl & boss type girl and other for Solar Buster cannon group.
It may sound silly, but this is the way to clear it.
Example girl for Kerria group is girl like Ivy, Stock, Mistletoe etc.
I have none of them, so I just use all dodge girls I have to tackle and pray...