Note: I just found this trick from the web.Credits wont Go to me. :D
But +Rep if i help. This must work because i tested this today. Use it wisely.

Requirement :

• Browser ( Firefox )
  
• Charles Proxy (Download)
  

Steps:

• Open Firefox browser
  
• Open Charles
  
• Start the Ninja Saga game
  
• Save your progress by buying a kunai or a Sai (The Sai is the best
  one.)
  
• Open Charles and search “CharacterDAO.saveCharacter”
  
• Right click and click Edit then click the AMF tab
  

• Scroll down until you find Inventory. Expand it. Search and then
  Expand Weapons.
  
• Edit the wpn1 or wpn 3 or whatever number into one of the following
  numbers to change the weapon
  

wpn2 = lvl2 Sickle
wpn3 = lvl3 Sai
wpn4 = lvl6 Tonfa
wpn5 = lvl19 WIndmill shuriken
wpn6 = lvl10 Battle Umbrella
wpn7 = lvl17 Ocean Sword
wpn8 = lvl18 Tri-Star shuriken
wpn9 = lvl70 Shark Skin sword
wpn10 = lvl60 Decapitating Sword
wpn11 = lvl9 Tailed Jutte
wpn12 = lvl7 Pointed edge Kunai

• Execute , Refresh Firefox/Game
  

• 
  
  
  
  • The items that you have buyed remains in your storage
    
  • Now you can sell or just buy an expensive weapon for 900 or 250
    coins
    
  
  </li>
  

Use this cheat at your own risk. Don’t blame me when you get banned.