Why can't Mirajane use magic for long periods of time? 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?
because you see multiple people run out of magic power throughout the show and she won't want to overdo it and be so exhausted not to move like Erza in the edolas arc and she might have finished the tartaros people off fast or with help