Ayra -spd or -atk?

I'd use +HP/-Atk. Ayra's most important stat is her speed.

-atk is better than -spd, since -spd will deny Ayra of some doubles and decres Regnal Astra's dmg

(deleted, too hard to understand. Check my other answer.)

Forget my last answer. Read this.
-If she has -atk, fix it with atk+3 sacred seal and she will have 49 atk/40 speed
-if she has -spd, fix it with spd+3 sacred seal and she will have 49 atk/40 speed
^ Both, have the same speed and attack despite their banes, so it does not matter which one you pick... unless you look at this:

The boon is the only stat that will change between the two. Do you want +hp or +res? I would go +res because +hp does not help desperation, and +res might let you survive a magic user, who knows... this might save you in an arena run or GHB