This is a Fire based Ninja build, using the uchigatana, infused with fire, along with pyromancies and the black leather armour.
Fire Uchigatana, Pyromancy Flame. (Other katanas will work, ajust your attributes accordingly. They also need to be infused with flame.)
Thief Mask. Black Leather Armour, gloves, and boots.
Flame Weapon. Warmth. Poison Mist or Toxic Mist.
Buff your Fire Uchigatana with Flame weapon to further augment the fire damage. Then, sneak up behind opponents to get as many backstabs as possible. Dodging is a MUST! This is because the black leather armour set doesn't provide much pysical protection.