To the topic at hand, a metaphorical fox vs an actual giant wolf.
To start this off, Ace massively overtakes her in the skill department, or at least the know-how to deal with giant beasts like her. In his first life of his over 2 millennia "life" span, he was a Roman, so he would have been conscripted to the battlefield some time in his life. In such battlefields, Romans had to fight off against/herd and use elephants in battle, and those bois are bigger than Holo here. His extensive knowledge would allow him to easily think of ways to defeat Holo. With that out of the way...
The very big problem he will have here is that he's practically empty-handed. Not only is he outranged if he goes into fisticuffs with her, but one wrong mistake, and he'll be unable to escape Holo's paw or hold or what have you. Even though he has the AP advantage (his casual 70.8 KJ compared to Holo's 50.5), there's not much he can do to adequately attack her before she does anything substantial. No weapons lying around, and any other item he can find in and around Central Park wouldn't be enough against a behemoth like that. Her combat speed would be equalised to his, but she can still run fast enough to catch Ace if he so chooses to run away. And don't get me started on that LS diff. Don't get me wrong, Ace is an exceptional fighter, and can dodge and weave around her attacks, but if he gets hit at least once, it's highly likely she'll get to use her LS stat well, and he'd be in her mercy. Either that, or I'm heavily overestimating what Holo can do, since he still has the upper hand in AP/Dura.
Most of what Ace has on him can only be used in Rider Form, so he's limited to throwing them at her to deal some sort of ranged damage. It wouldn't be much, but they would hurt, I guess.
And he has canonically done it once, too!
His Supernatural Luck can more often than not save him from hairy situations and hinder Holo throughout the battle. Protruding roots can aid in tripping and slowing her down, beehives can suddenly drop onto her from the trees, and just about anything in the surrounding that can be metaphorically thrown onto her face. But it can only do so much before Holo just powers through, dodges or something and tag Ace. His Desire amp would be very potent here, since Lamentation arc was where he was the closest to fulfilling his desire of meeting his mum, and because that opportunity slipped away from his fingers, he'd be more determined than ever to win a match like this. This wouldn't be much of a factor in usual hand-to-hand fights, but in Ace's eyes, he's fighting against a beast in what he perceives to be a life or death fight, not some martial artist that would knock him out or something. Desire Amp would kick in almost immediately, and he'd equalise any lower stats he got to Holo's, and mitigate any weaknesses he'd have here. Although, I know this shit would be contested (which I'm willing to debate lol).
There is one very impactful "weapon" he has against her; the Boostriker. Thanks to its
subsonic speed, he'll be out of Holo's attack range due to outspeeding her tremendously. Not only that, but he can also
"jump" into the air (or onto walls if they make it to the city) to dodge attacks and/or ram against her where she can't counterattack (like driving behind her legs or neck and ramming right at those places). This is the only use his Buckles (specifically the Boost Buckle) will have
without transformation, beyond the unconventional ranged attack. Though, his summoning takes a while, and Holo might get a hit or two already, or even worse. But this is usually when supernatural luck comes to save his ass, so there's that to consider.
The better and more reliable answer to that is his
limited teleportation! It's only limited because he can only teleport to and from
this specific location, which is god know how high up in the sky. Given how he can just "materialise" his belt from thin air, this motion to teleport is way faster than you think. Then he can just drop from there riding his bike, and we have him on his Boostriker safely without the interference of any grubby paws!
Basically, he'll have problems fighting Holo normally, but he'll have more than what it takes to take her down when on his bike that he safely deploys from somewhere Holo has no hopes of getting to
Figured the 9-B thing clarified that, but I will add it more explicitely to the OP.
Reiwa Riders now have the tendency to have their main rider weapons in civilian form, and they count as Standard Equipment. So it has to be clarified.