Even if Age Seal can trap Acnologia in time, Dimaria lacks the power to take down Acnologia in human form. In dragon form, Dimaria lacks the power to land even a single scratch. She can trap him in time until her power runs out, but that's about it. Mages like August are as powerful as Irene, and he knows every type of magic. Zeref too is more powerful than Dimaria, and Zeref can also stop time. Zeref, Irene, and August still recognize the need for Zeref to obtain Fairy Heart to obtain heart to take down Acnologia.
Acnologia is a dragon slayer, and this is what dragon slaying magic does:
Probably not much. Acnologia didn't flinch when Igneel tore off his arm. That's about as painful as a wound can get, and it was an attack that actually did physical harm to him. Plus, age scratch is used as a response to someone touching Dimaria. Dimaria would be dead in first contact.
There are two possible ways due to which Dimaria wouldn't be phisically capable of ending Acnologia:
1. Because Age Seal can't trap him in time. He exceeds Dimaria's and Age Seal's competences due to whatever reason (it doesn't affect entities with more MP than the prey, for example).
2. She manages to trap him in time, but she cannot hurt him because his skin is too durable.
Mashima has got us used to some patterns throughout his manga. One of them is that the stronger the mage, the more MP he generally has, and the more durable he generally is.
By that token, August should've been so durable that his skin should've been impenetrable by any blanded weapon whose strenght does not surpass his durability. Brandish is a hax-y magician, not a CQC specialized magician.
However, Brandish was capable of breaking through August's passive durability. How can we account for that?
Probably because durability is more relative than we want to believe and, except for some instances, such as the one
@Shoutmon very well pointed out using the example of Natsu and Wally; passive durability is not governed by an only rule (= the stronger, not always the more durable).
In his human form, Acnologia seems to be the strongest, fastest and most durable 'human being' in the manga. However, he doesn't seemingly preserver his full Dragon aptitudes. I could see why many of you would agree Dimaria has got nothing in her arsenal capable of hurting Dragon form Acnologia. However I'd like to abide by possibility 1 above. I think Dimaria should definetely be capable of actually hurting/slashing/piercing through human form Acnologia, it's simply that she won't have the chance because AS is not working on him, so she's not getting anywhere near her.
Anyone who chooses possibility 2 is as entitled as I am to argue on that. After all, both possibilities are mere assumptions.