3 things you will need:
-Pow stat & title for max affinity.
-Enhance Weapons with higher ranked skills.
-Skill Force Magatamas.
Having a good title from Amaterasu helps a lot. You will want Date Masamune title (+60% affinity to Swords, Axes, Odachis). Requirement is to have a level 60 Kagehide
http://onigiri.wikia.com/wiki/Title. Just dump your stats into POW for now until it's maxed out to 2000 affinity total with the title. Generally it is easier to concentrate on a single type of weapon when starting out. But if you want to play multiple weapons, POW build is easier because Odachis, Swords, and Axes are all POW based. If you have Blue Moon Blade, try to reach the damage increase milestones for POW then either Vit(more HP) or Dex(crit+reduce cooldown).
Spamming skills is usually how people level up. The higher rank the better. You want skills used for 1hit KO single targets and mobbed targets. Depending on what kind of damage you are doing, you may opt for lower ranks if your sp can't keep up. Spam SP potions, they're pretty cheap. You can rank up skills by smelting with Yoshitsune and selecting the rank increase option.
Having good mags make a lot of difference to your damage output. If you have trouble finding them, get Miroku to at least level 16 so that you can just buy multiple Yoake Mags
http://onigiri.wikia.com/wiki/Yoake_Magatama. It only affects level 3 and above skills. Slice helps to increase damage too.
For more information, you can check out this topic:
http://onigiri.cyberstep.com/_/forum/viewtopic.php?f=12&t=7939Edit: Topic previously answered by Muffin and Helix.
http://onigiri.cyberstep.com/_/forum/viewtopic.php?f=17&t=8927&p=33330#p33330