What A slot should I get for Maribelle?

Brazen atk/spd, if you are going for an offensive healer then brazen atk/spd is the best choice.
otherwise the spd 3 would be best way to go.

Both are fine, she will be great. Now, I wouldn't recommend a brazen skill, I know why they are good but healers, specially wrazzle dazzle ones, shouldn't be taking damage in the first place to proc it. Some are capable of by tanking magic but a res bane Maribelle won't.