Define "buster".

Is this about sustained attack power? Because Luffy's been at "destroy a city block" level from at least Alabasta arc, and "annihilate a building" level from at least Arlong Park. Factoring in his stamina, if left unchecked, he's been able to destroy a small city from the word go.

Or is thus about single burst power? Because Zoro could cut down a giant-sized stone opponent and Luffy's comparable in strength. In Fishman Ark, Luffy's barrage of attacks was doing quite a number of that giant Ark. So definitely "city buster".

But "continent buster"...? Yeah no. No on-panel evidence for single attacks of such strength. Sustained attack is a different question. Left unchecked, almost any heavy hitter in One Piece could wreak havoc across a continent.