Which Myrrh to go for.

For flying Myrrh? +Def-Hp. You do not want her to tank anything else then physical attacks anyway. So go with +Def.

Definetly +Def-HP, the only one better than that would be a +Def-Spd.
+Def sinergyzes godly with her breath, and its still more damage if you have bonfire/ignis, and more tankyness.