The Odachi. Smashy/slashy, 40+, needs Big Choppy Weapons 1, bonus from strength, 155 base power, no auction. The design is really nice. As you can probably tell, it's a Buccaneer weapon. Though the class isn't required.
I know there is one weapon you can get from the Bazaar in Skull Island that is two thin swords that you can only get if you are, like, at least level 45, or something like that. I think the name is something like "General's Swords"- I don't really know, but I used them for my Swashbuckler for 10 levels because they were so good (for my level at that time)