IMO, Aeron can wait if you have Costume Rigard. Athena is great sniper offense and defense.
She is great after she get buffed.
Here is some discussion about her Is Margaret useable now with the buff?
Yes, and Brynhild is the only one healer for trial of survival quest.
EDIT: I forget, Puss in Boots is also healer but PiB is harder to get.
As I see you have his costume, and he is great with costume, defdown all ooponent in 9 tiles (if you have Mana Troop lvl5).
As you already have wilbur, I think Sonya is more important as she is sturdy fast dispeller.
If you already have Isarnia as defdown blue, I think Sonya 1st before Grimm. But both are great.