Best places i foundt out to level a 56 sam depends on your subjob. If your subjob is DNC then i suggest you go to The Santuary of Zitah, or, if that aint working for ya try Western Altepa desert. Both locations have a Field Of Valour book, where you can get tabs, extra experiance field suport and additional gil.
If you have the expansions, try to work on besieged and Campaign, altho due to the recent updates i think the experiance there is gimped.
Last but not least ( wich i found out ) dont be afraid to party and dont be afraid to sync level. Your skill ups on Great Katana will eventually come, and you can always skill up later in skillup parties at The Boyada Tree.
Draigon (sam75) Asura