The enemies he faces are always stronger then Goku. The thing about Goku is as he fights he gets stronger. That is because he never uses his full power until he absolutely needs to. His opponents are to powerful and smart to just be teleported to the sun. Look at Whis, the guy who trained Beerus who is stronger than Goku and beerus only used %80 of his power and Goku could not beat him even asa God.