Dazzling Pain

Nanna has significantly better survivability than Mist. High spd makes her hard to double, and her bulk is actually decently balanced to allow her to survive one hit. For most healers that's as much as can be expected without significant investment. Nanna will also be better at charging miracle because she will double any slow opponent.

However, since you've already given Mist pain+ I would just keep her. The effectiveness of pain+ compounds or snowballs as there's more of it, and dazzling staff makes Mist's low spd irrelevant during her own turn.

You don't always need Wrathful Staff, especially since they both have somewhat low Atk. Even with her Spd, Nanna is still great with Pain because she'll most likely poke an enemy twice and hit them with her Pain/Blow. Miracle will go down to 3 as opposed to Mist bringing it down to 4, which also helps with survival. Since Mist is slow, activating Miracle can backfire if she gets hit twice and it activates on the first hit as opposed to Nanna who is more than likely to only get hit once.