@DBZ_universe: Well, Chaos (the galaxy's source of evil; the final enemy in the SM series) was able to cause destruction and ruin to the entire galaxy, and Usagi was stronger than it. And okay, I get the space breathing thing. Usagi can breathe in space, too, anyways, so I guess it doesn't really matter now.
But Sailor Cosmos has time and space manipulation. She could travel through time and send others through time, as well, with no limits. Also, since her Cosmos Crystal allows her to use the powers of all of the Sailor Soldiers, then that means that she can stop time, manipulate dimensions, etc., since Pluto was shown to be able to do such things. However, Sailor Cosmos has no limit to her powers like Pluto does (Pluto is restricted by three taboos that are there so that she can't disrupt anything. Breaking the taboos will result in a punishment for her), so she can do whatever she wants with time and space.
With her time and space powers, Usagi could do a lot of things to defeat Goku.
A) Freeze time and kill him.
B) Send him to the time of the Big Bang.
C) Send him to another dimension and seal it off forever. This would make it impossible to open or destroy, even by Sailor Cosmos herself.
D) Send him to the Fourth Dimension.
E) Travel back in time and kill his mother before he's born (this one's horrible, though, and Usagi would never do this if we're putting in morals).