What am I supposed to do with a +Atk -Def Sakura?

Without wrath or dazzle or candlelight only cav healers can hold their own in arena and even then it is a wasted slot unless they are heavily merged.
Sakura might be alright for tempest trials if you can get absorb or put renewal on her. Good for chain challenge too. Healers really need rehab though to be much good in the upper levels.

If you have a spare Dazzling Staff you could run a debuffer Sakura with Fear, Panic or Slow. If it's it's possible to inherit for her, Panic Ploy as a C Skill could be useful paired up with Fear, Slow, Panic or even Pain to cause some problems for the enemy team if they try to buff their team up. I would try to give her Fortress Defence to patch up her Defence because of that Defence bane otherwise she'll be obliterated if a fast melee unit gets too close to her. You'll lose a little of her attack but her job will be to debuff the enemy team and retreat safely.

Nothing. Healers suck. They don't do nearly enough damage even with wrathful staff, they can't have offensive specials, and they score poorly in the arena.

If you can afford it, slap Attack +3, Wings of Mercy + Rehabilitate, and Fear or Gravity, or simply Assault.

Done. You have a nice anti situational Rehab healer that has the potential to finish off her enemies.