According to ONE, the author of Onepunch Man,
Saitama's power when he gets serious is equivalent to the Big Bang.
So yeah, one punch of his can send the whole universe to hell. If he ever seems to 'struggle' at all is because he isn't taking things seriously.
In any case, can still think of a few character stronger than him and that is, people with Reality bending powers.
Nagato Yuki for one, who can overwrite reality,
Heaven Ascended Dio who can erase people out of existance, Goddess Madoka who is ina constant state of destroying and creating new universes.
So, anyone with God-like, Reality Overwriting powers can defeat Saitama without actually having to look at him.