Whis has stated that he is stronger than Beerus, and Whis once proved this by knocking Beerus unconscious with a single hit to the neck. He can easily take on Goku and Vegeta at the same time and claims to be the quickest and strongest in the universe. However, it should be noted that Whis has been shown to be lying about his strength on multiple occasions. For example, when Krillin tried to fight him, Whis showed him how weak he was by blowing him away with a single punch. Also, when Wujing saved Tao from being killed by Beerus, Whis claimed that he could have destroyed both of them but decided not to since they were friends. It's possible that Whis knows how weak he is and uses this as an excuse for why he doesn't fight back. As for his power level, there are two ways to look at it. First, we can consider his own body to be stronger than Beerus'. This is proven by the fact that even after taking a blow to the neck, Whis still had enough energy left over to knock out Beerus with one punch. Second, we can consider Beerus' body to be stronger than Whis'. This is because even after taking several blows to the chest from Whis, Beerus was able to block them all without any problem.