Remember when Luffy fought Spandam, because he was the boss? :saden:
The Gorosei might be strong, but not all of them are going to be at the highest level. Saturn being the first to be deployed, means he's going to be one of the weaker ones. That's how Oda operates. Luffy will one shot him to replicate punching a CD on Sabaody, and then move on to fighting Kizaru, an actual Top Tier, to get his revenge for losing his crew on Sabaody.