Cracker coa>Doflamingo coa!Cracker fought luffy for hours!Manga stated Cracker was the most troublesome foe Luffy had faced at that point so Cracker>Dofla!Pica never showed Ryo(coating rocks with coa),anything Zoro did,Luffy could do as well to Pica!Pica just had an annoying devil fruit!G3 counters Pica's casual attacks,G4 would make short work of Pica,because G4 can fly and put immense firepower in a short span of time!Rocks ain't that much trouble for Luffy,man!Pre-skip Luffy without g2 and g3 sent crocodile flying from the underground in a similar way Doflamingo was defeated,he destroyed many layers of rock!Luffy Break rocks from any size all day,mountains,islands,you name it!
I know Tesoro is not cannon,but it would give you an idea how Luffy vs Pica would play out!Pica would never beat Luffy!Period!G3 gatling gun or G4 would shatter Pica's Statue into rubble and he would be defenseless!But i do agree that like Cracker,Pica can stall Luffy because he is hard to get to!