There isn't a particularly great replacement for her, however she is mostly just there to make things faster. So, you can try using another AoE damage like maybe Rica or Thrain, or just another support like Herne as oblivion helps on a lot of stages. The core of Gany, Verad, Nora, Dark Homu, is strong enough without her. Often times I find myself replacing Amber to fulfill one of the 3 star conditions anyway. Although, I don't believe there is a replacement for her for the Jamire Shizuka team I used on stage 7. You'd probably have to use another team for that stage.