Izuku's only real chance is to take out Mash quickly with Gearshift + Fa Jin combo. With his blitzing level speed he could land multiple blow on Mash with Fa Jin before he adapts, but I'm uncertain about that. There's also the possibility of him charging Fa Jin in multiple limbs as well, so he can use those extra hits just in case.
At max I might say Izuku can charge up his full Fa Jin into each of his limbs, giving him four attacks at 41+ Teratons before having to charge again.
And that assuming Izuku gets the chance to, I can't say for certain if he'd continue wail on the guy after hitting him dead on with just one hit. And I'm not fully certain if Izuku has the control to hold Fa Jin in all of his limbs like that. It's just an assumption so it's not very concrete, the most he did was two limbs.
Gearshift alone can't beat Mash, Izuku's AP isn't high enough and he'll just adapt to that level of power. The fight ends if he outlast the five minute limit.
Honestly, IDK much about Mash but going off the profile there's a very high chance he'll take the hits and end up adapting so Fa Jin + Gearshift's power. Also, I think Mash's combat ability surpass Izuku's by a large margin. His ability to predict and analysis his opponent is insane.
I think Mash adapting to Gearshift + Fa Jin's power and outlasting the five minutes is actually more likely than Izuku taking him down in that same time. To be fair, once Mash gets a chance to adapt he can take those hits with far less damage than he did against the initial strikes.
Voting Mash for now.