Q is one of those characters that, on the one hand, often gets people invoking the no-limits fallacy based on the implications of what he could do. On the other hand, he tends to be underestimated by people who only picture two guys throwing raw power at each other.
I don't know how valid this is against Goku but Q, without artificial game rules restricting him, just goes back in time and makes it so someone just never exists to begin with.
We've seen Q do this. Nor does it seem to be an alternate reality. He just remakes the past. In fact, he juggles multiple realities and timelines, making them interact or not interact as he pleases.
This is more about how Rumbles rules may or may not restrict him than about his abilities.