Yes and no. She actually moves when you get near. You can move to the forest to the left side (where Aideen and Dew come from), Cuan would do, use him as a decoy to lure Ayra then escape a bit and strom the castle, seizing it, and then talk to Ayra with Sigurd to recruit her, that will do. This decoy is tricky sometimes, because if Cuan is a bit far from her and the other units that will storm are close, Ayra will go after them instead.
As for Arenas, there are 7 enemies in it, you can fight them during the entire chapter, but using your weapon (it drops its durability) and also what I pointed out before of HP. This time, you don't lose units, but their HP goes to 1 (if I recall correctly) if you lose a fight.