I think your arena team is good as it is. 

Coldheart for dungeon content would be a solid choice as next 6*. I recommend to use a hp% chest on her, otherwise she is dying all the time, her defensive stats are ridicolous. The damage of her A3 has a small part scaling from her attack (1.7 x atk, that's laughable anyway), but the main part scales from enemy max hp and is increased by crit damage.

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That's how I built mine. 296 accuracy to land the tm decrease without problems, enough hp to survive a hit from FK 25 and enough crit rate to get a critical from her A3 all the time (30% extra chance on this skill). Helps me much more than the high attack build I used on her before, because every random enemy from the FK 25 waves could one-shot her. And the normal damage of her attacks is so low, it's not worth investing. 0.7 x attack on her A1, even as a 4-hitter that adds up to 2.8, and I allready mentioned the 1.7 x atk from her A3. Not worth increasing with an atk% chest.

You would never go wrong with Scyl or Geomancer. But the quesition is always where do you want to improve. Personally i would probaly go with Scyl she is good in almost any instance. Even Arena. in defensive teams.