It's bad form to reply to your own thread, but what the hey..
Groo beats Usagi, imo. He single handedly took a city that was considered untakeable in the first issue of his 80's run. Groo is both the stupidiest and toughest barbarian out there.
Now, Usagi could easily outsmart Groo, but imo he won't think to do so because he's honorable.
While his friend Gen, otoh, would outwit him every time, as he has Usagi himself.