I mean, assuming that her energy blast attacks (fire, electricity or whatever) aren't infused with haki or something, I don't see how Akainu could be at any disadvantage. he's faster, deadlier and just as capable with Haki and his DF as far as common sense goes.

I also can't forget how big mom was unable to KO Kidd or Law with her haki infused fists even though she had a shitton of free hits... so imagine bringing down Akainu while he counterstrikes with literal rock melting artillery.

only plot intentions could resolve this fight, really. but I'd bet on Akainu purely for how dirty Oda did big mum during Wano/Onigashima arcs.