User blog comment:IamJakuhoRaikoben/Chapter 493: The White Dragneel, Review./@comment-76.161.172.226-20160711151314/@comment-76.161.172.226-20160711153800

The thing is Mirajane wouldn't have struggled against Irene, she would have just been flat out stomped. She fell behind Erza and Laxus years ago when Lisanna "died". Even since she regained the ability to fight, and Lisanna returned, Mirajane continued to bar-tend and wait tables rather than take missions and train. She's spent most of the series closer in strength to Natsu, Gray, and Elfman than to Erza, than people have been willing to admit. Even with her putting in a bit of training over the latest timeskip, it's pretty much guaranteed that Natsu and Gray have surpassed her significantly. I wouldn't be surprised if Elfman has as well, seeing as he spent the whole time training.

So if she just fought Irene and gotten stomped, she would have looked like a weakling who couldn't even take on minions. Defeating Juliet and Heine makes her look reasonably strong, and leaves how badly outclassed she is by the Spriggans up to speculation.