The God Of Procrastination said:
Weaknesses: He doesn't take anything seriously initially, and has a tendency to hold back to possibly prolong a fight. Saitama has no true martial arts training, as he simply just fights, and is possibly unable to breathe in space. Once she realises that punching won't work, she can take them into space.
So are you voting for Tsubomi?
By the way this is how I think should go:
(1) Saitama punched Tsubomi. Tsubomi died.
(2) Or, Tsubomi survived, Tsubomi punched Saitama back. Saitama did not even flinch. Saitama punched Tsubomi with a much bigger force than before (getting more serious). Tsubomi died.
(3) Or, Tsubomi survived and repeated (2).
(4) If she was lucky enough, she could punch Saitama into space. If Saitama cannot get back on Earth fast enough, he died out of suffocation or Tsubomi punched him to death in space.
(5) If Saitama came back to Earth, Tsubomi could stay outside space and Thunder McQueen witnessed a treaty of truce.
(6) Or Tsubomi tried (4) again, but Saitama either punched Tsubomi with Serious Punch killing her...
(7) or Tsubomi succeeded punching Saitama into space again, repeating either (4) or (5) or (6).
So more times than not Saitama is winning by sheer AP and durability advantage.