Yeah, I could definetly see Buggy fighting plenty of Marines and other pirates; he might've even taken out someone with a big bounty or high rank for all we know.
While fighting marines and defeating them could very well be true, even captain level marines are nothing more than canonfodder and aren't good representations of battle abilities. In terms of Buggy fighting someone with a big bounty or high rank, i find this hard to believe. Buggy was the 1st pirate introduced in East Blue, and also has a relatively low bounty, this means that he keeps a low profile and doesn't challenge any pirates with high bounties, nor does he challenge any marines with high ranks.
Except Alabasta was protected by Crocodile and Drum was ruined by BB crew.
And I don't see where the "Buggy's getting stronger by sailing GL" stuff comes from. At his age do you really think there's any room for improvement? It's more likely we haven't seen his real combat abilities yet and that he's making good use of them to currently survive on Grandline.
Also Buggy will always be a big threat for Zoro as Zoro can't hurt him.
Good point. As I've been saying, I doubt Buggy has made the kind of improvements the SH are making. I've considered the point that there is definitely more to Buggy's abilities. My problem with believing that there's anything grandscale is when I try to size up Buggy to the likes of a Shichibukai, a Logia user, or someone of Lucci's caliber. To me, Buggy is someone like Foxy, who has a great potential but just doesn't use all of it.
Zoro vs Buggy - i think this would be a good change of rivalry in the show. If zoro could find a way to harm Buggy, i think that'd put Zoro on a whole new level, but sometimes the best solution is the simplest, and Zoro could simple not use his swords and instead his fists, then Buggy is now succeptable to all attacks.