Michael is a lot more experienced than Franklin, true, but Abby is still a better fighter than Mike, and maybe the verse as a whole outside of CJ.
Michael has a much larger, much better arsenal of weapons, and has the AP and speed advantage, and being able to slow his perception of time, along with being a better gunman makes it extremely dangerous for Abby to engage him in a direct fight.
But she's got no problem avoiding that, and getting around it. She's been able to take out enemies who outgun her like Ellie and enemies who are stronger than her like the rat king.
I wish you could have been a little more specific on locations, but still liberty city gives her plenty of areas to hide around. She has enhanced hearing to be able to hear and detect enemies through walls and things like that, she could just rig an area with pipe bombs and could craft more on the go very fast.
Or Spray Mike with a flamethrower, because none of the HD protagonists are fire resistant, or just snap his neck with her superior LS, If Davey could pull off a sneak attack on Michael, Abby definitely could.
Short summary
Abby:
+Stealth capabilities
+Flamethrower and Molotovs
+Innovation
+Lifting Strength
Michael:
+AP advantage
+Speed advantage
+Larger arsenal
+Higher quality arsenal
+Time slow
+Superior marksman.
Mike has more advantages, and if Abby even makes the slightest mistake she's dead as ****** fried chicken, but she's been in this spot so many ******* times to lose here.
The odds were heavily against her favor against Ellie, and against the rat king, but she won those fights because of the very advantages she holds in this one.
And given the location of this fight, they're bound to play out again.
Abby High diff