Depends on what you expect.
I doubt Oda would let Luffy's underlings beating Admirals casually, I even doubt Luffy himself will defeat an Admiral or not, short clashes with Admirals for sure.
If the marines will be defeated that should be via the revolution and through the upper levels of the government changing and putting the right Marine on the job. A pirate beating an Admiral resolves nothing, they are an organization, beat Admiral at best that Admiral gets demoted like Magellan got demoted from being Warden, and they replace him with another Admiral, organization must change.
Which is why I don't see why defeating an Admiral should be important, being stronger than them is important, beating them is not you can prove superiority without beating them.
So the most important opponents for them will be pirates because this series is about Luffy fighting against Shichibukai first then Yonko, its not about the Marines or the Government.