To make the Ninja in Little Alchemy, you will need to combine the following elements: Human + Sword.
The first step is to create the Human element. To do this, you will need to combine the following two elements together: Life + Clay.
Once you have the Human element, you then need to combine it with the Sword element. To create the Sword element, you will need to combine Metal + Fire.
Once you have all of the necessary elements, simply combine Human + Sword together and you will have created the Ninja element in Little Alchemy.