So are you agreeing with me that Mirajane lost the ability to do most of her magic when Lisanna died?I think it was her 2nd fight after she recovered the majority of her MP. So it very understandable for Mirajane why she cant use Sitri at that time. But like what she said there is a possibility that she can beat Azuma if she will channel her remaing MP to attack Azuma but as much as want to. Lissana's safety is her priority.
I realize I missed a key detail in my last post on why Erza outperformed Mirajane on the Tenrou arc, so I'll flesh everything out here. Mirajane presumably used all her magic power to protect Lisanna, so she took Azuma's hit without her magic protecting her. Azuma's blast knocked her out. Erza's cherry-blossom armor
puts all of her magic power into attack power leaving her with zero defenses. That means' Erza also took Azuma's blast without any magic power protecting her. Furthermore, she took a blast enhanced by the full power of the Tenrou Tree. She gets back up from this. Then after she defeated Azuma, she went on to fight Hades. If you're saying that Mirajane used up so much magic power fighting Elfman and Evergreen that she couldn't muster up the strength to summon Sitir....Well that would be like Erza using up so much magic power fighting Juvia and Lisanna, and then be unable to summon her strongest armors due to a lack of magic power.
Could Mirajane have beaten Azuma on Tenrou? Probably. Could she have beaten Azuma after getting blasted by the Tenrou Tree with zero defenses, and then after a short rest go on to fight Hades? Probably not.