So, later on in the series, during the fight against Zeref, we see Mirajane use Satan-Soul but she also used souls that she had collected during the Tartaros arc. My question is: why can't she fight for long and how did she collect so many souls if she can't fight?
My theory is mirajane's personal preference/Personality is as such!
In the starting episodes of Fairly Tale, we only saw her as a simple girl. During the episodes about Lisanna story, we also learn that she used to be aggresive but after the Lisanna death she changed completely.
As I understand, Natsu though trainings improved his strength and stamina but mirajane did not improve as much.
(During the initial episodes) For me the shocker was when we learn that she too has magic and that too very power stuff (S class level). But, I see that most of the time, she preferred being a sweet girl, who enjoyed modelling and take cares of her friends.
(Also the story is about Natsu, if mirajane is too powerful it would overshadow lots of stuff :)