This is actual the only reason why Luffy won't be a push over. I think injection shot will somehow weaken Doffy. Something like that hast to happen, because 1 vs 1 Luffy can't win. Sure, once a elephant gatling hits Doffy, it could be over, but Doffy isn't that easy to hit. Both sides have one hitter moves, well now while i think about it, brutal physical force wouldn't knock out Doffy that easily. Doffys power is more deadly and effective. Plus he really is physical strong and he can use busou and kenbun at least as good as Luffy.
Plus, i guess, we haven't seen Doffy in serious rape mode :/. And having in mind that elephant gatling is Luffys strongest move, it has to be, because he wanted Noah gone asap, i can't see a victory on Luffys side. Luffy will hold his ground and then baam…(Laws sheming smile) my Injection Shot wasn't just a stab bwuhahahahahahaBWUHAHAHAHAHAHA...drops dead.
Injection Shot was a ranged stab as it caused Doula to bleed when the sword wasn't touching him.