Oh, well, asuna would need to know how it works first, so the fight would go on for a bit before that, but asuna will eventually try this. Personally, i don't know what she'd come up with, especially since she has higher battle iq than myself, but Asuna might take inspiration from some of her already existing skills, like luck magic, Or, More likely, from others, Such as Chidori, Who used nullify to counter asuna's luck magic when she tried (and failed) to use it to beat her in a board game. Shizu also used it in the latest chapter, so it's not too far-fetched asuna would try to use it herself to temporarily disable spag's defences and get some hits in. I didn't bring it up cause i wasn't sure if asuna would get the opportunity to do this, but since she can buy herself time now that i think about it, here we are.
If asuna did manage to learn nullify on the spot, she can use it to disable any singular thing spag is using for a short period, such as the red circles, but asuna will go for his defenses, more specifically, the digi-shield forcefield around him. He's still partially protected from asuna's blows with his armor and ranged damage reduction, but asuna would be able to make some sort of progress with this, even if it's just a dent at first.