Shanks’s fighting style is likely a mix of both Roger’s and Garling
We already know he was the only Roger pirate that was able to copy divine departure. Also like Roger (and the others that wore the strawhat) he also looks to be a haki master
Also I personally believe Roger might’ve been a monster in multiple fighting styles not limiting himself to one way of fighting. Him maybe getting the name Anne from a gun he had. As for where is it idk probably the same place Blackbeards claws are
After shanks lost his arm (dominant one at that) he’d obviously not be as strong as he once was or at least could’ve been with 2
For Garling based on his son shamrock and the gorosei he likely uses devil fruit powers. Maybe in his case he has one in his blade and one he himself has
Shanks like his brother might also have a devil fruit sword. It is named gryphon after all like shamrock’s Cerberus. What he’d lack tho is himself having eaten a fruit
So to the question of how shanks actually fights
- Haki master like Roger but worse basic fighting ability after losing his arm
- Devil fruit sword like Garling but no devil fruit eaten himself
- So pretty much his physical body is just worse than theirs due to lack of fruit and arm
