+Res -HP or +Spd -Atk NY Azura

i would go +Spd, she usually wont be attacking so -Atk doesnt really matter
HOWEVER if she is the only green unit on your team you NEED to give her TA otherwise you will have some trouble, if not, don't worry about it
Also i would advise against merging hone fliers is a rare skill to come by (the rarest hone skill, its only on 3 units 2 of which are seasonal) even if you don't plan on using flier emblem (you should because of the monthly quests requiring fliers) you really shouldn't be merging anyways unless you plan to make Airzura a permanent arena core member, in that case merges are everything (for tier 20 at least) but usually i would keep her just to have hone fliers if you ever need it

The HP- is bad if you're using her in Arena since it's one of her super banes with Res- being the other one. The Spd+/Atk- won't hurt her much since she's more of a support unit and also the extra boost in speed will help her avoid getting doubled too badly. I speak from experience since I have a Spd+/Atk- NY!Azura on my flier team.