Roger defeat everyone starting from pirates to Marines but I have a question, Can Roger defeat Imu-Sama? He played a major role in Whitebeard's death, and then went on to steal his Devil Fruit.
Luffy, the show's main protagonist, is the captain of the Straw Hat Pirates and a member of the Worst Generation.
Prior to his death, Roger entrusted his son to none other than Garp, telling him that he trusts the man just as if he were one of his own crew members.
The audience is well aware by this point that Ace loathed Roger with every fiber of his being, disavowed their relation, and hoped to claim the One Piece so he could make Roger's rival Whitebeard the Pirate King as thanks for being his beloved replacement father.
And all this after twenty years of non-action.
Now imagine a normal sized dude,a bit on the tall side, but still normal sized competing with these giants.