What's Mira's counter, exactly? Erza had her Neptune sword and her ability to blow sand away, but Mira? She was only able to dry up a nearby lake and, as far as I recall, there is no moisture in sand, so that would be useless. Even if she has more ranged attacks, how is she going to hit Ajeel in sand world, when she can't see him?
Her nukes can disperse sand with much less difficulty than Erza using her sword. Let's be honest here, when there is an explosion, everything including rocks and debris scatter. Mirajane can use this to her advantage by nuking the area around her which is how she will be able to reveal Ajeel's location.
I don't think Erza is slower than Mirajane at all; is there any evidence from the manga to support this claim? Please, provide evidence to back up your claims. And yes, Erza hit him before he entered Sand World, or when she was able to disperse the sand momentarily. In a full sandstorm, that is very different, so Mirajane is doing any better than Erza.
Well if you want feats, that will be hard to find considering Mirajane and Erza have rarely fought the same opponent. So it won't be easy to gauge their speed. Plus, Erza also has many more feats than Mirajane. The only scenario I can come up with is that Mirajane can keep up with Etherious Seilah who is faster than Etherious Kyōka. And Erza surely had a hard time against her.
Anyways, I think it should be obvious that flying is faster than foot. That is why Team Natsu always relies on Happy to carry them long distances. That is why Acnologia can blitz Gildarts without him even having a chance to react. Besides, do you really think Erza could catch Freed when he was flying? Mirajane did.
How does it outclass Erza's feats of being able to dispel magic itself? Clearly, you are extremely biased. And like I said above, there is no moisture in sand, so her MP or said "feat that clearly outclasses Erza" will not help her at all.
And no, I'm not underestimating Mira. She's definitely one of my favorite characters, but saying she could possibly defeat a Spriggan when she only defeated two subordinates? I mean, she had to use Alegria just to defeat mere soldiers. While that is certainly a huge power-up for Mirajane, it just shows the limitations of her power. She isn't on Erza's level yet, so she cannot possibly defeat a Spriggan, when Erza could not.
Erza dispelling magic is a moot point because she didn't even need to use it against Ajeel. So because we're talking about Ajeel here, Mirajane wouldn't need to outclass Erza in dispelling magic. Her MP was able to push the water aside (you can clearly see somewhat of a trough), so I would say it can push sand to. But even if you disagree with this logic, you can't dispute that Mirajane still has her nukes.
There is a major flaw with the argument that Mirajane is not Spriggan level because she only defeated Eileen's subordinates. I don't know why many people use this as some sort of basis to prove Mirajane is not capable of handling Ajeel. First of all, Mirajane defeated Juilet + Hynhe fairly easy after using Alegria Soul. Not so much with Erza against Ajeel. Secondly of all, Mirajane has this power for an unlimited amount of time according to the conditions. Something Erza didn't get with her Nakagami Armor in her fight. Now if this isn't downplaying Mirajane, I don't know what is.
Let me give you similar analogy. Natsu used a base punch to hit Ikusa-Tsunagi, yet there was no effect. The he used an FDK spell to ultimately defeat Ikusa-Tsunagi. Would that be evidence to go against him when comparing him to a Spriggan? No. Why? Because he performed that feat with ease.
If Mirajane defeated Juliet + Hynhe with extreme difficulty, then I would agree with everyone's arguments. But it seems like we're just assuming that whoever beats a weaker opponent is automatically weaker than someone who beats a stronger opponent (without even considering the difficulty of the fights).
Erza's fight was extreme difficulty; the fact that she almost lost her life proves just that. But, like I said, I don't see Mirajane replicate something like that, when she hasn't shown any counter to Ajeel's strongest spell...
Mirajane is not going to be a poor match-up against Ajeel like Erza was, so why can't she replicate Erza's feats? What is Ajeel's strongest spell?
Sand World? I already said how she can counter that. She can even counter
Sands of Death,
Ant Lion's Pit, and
Sand Golem as they are all ground based.