I've never been beaten by this boss but i guess Grand Cross is a little annoying, what you do simply is having the abilities auto-haste equipped on all characters (especially on the healer) and auto-life, (If enough then Auto-Regen as well) if you don't have it then i suggest leveling up more.
Steiner should be in your party using Shock or something like that, Vivi can use Flare (Reflectx2 perhaps), or Eiko's carbuncle/auto-reflect, your choice.
If you're having problems then maybe Eiko is more suitable then Dagger simply because she has better supportive summons and offensive magic.
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