A ninja can do significant damage by utilizing what is often called the "elemental wheel". The elemental wheel is accomplished by casting the -ton spells (either the level 15 set ("Ichi" meaning "one"), the level 40 set ("Ni" meaning "two"), or the Merit level set ("San" meaning "three").) in rapid succession, keeping in mind that not only does each one deal damage, but each one lowers resistance to a specific element for a short amount of time as well.
For example: Casting Hyoton deals Ice damage to the target, and then lowers their resistance to Fire. Casting Katon after this would be your next choice, as this spell deals Fire damage and lowers resistance to Water, and takes advantage of the lowered resistance from the previous spell. Next Suiton would be cast, and you'd follow the elements around the wheel and complete the cycle. A simple way to remember the casting order is to simply remember to follow up the previous spell with the element that the previous spell's element would be weak against.
- The full cycle from an elemental perspective is: Ice > Fire > Water > Lightning > Earth > Wind > Ice
Note that this is a different cycle than simply the weaknesses associated with the Elements.
- The full level Ninjutsu cycle is: Hyoton > Katon > Suiton > Raiton > Doton > Huton > Hyoton
Due to long casting times and low damage, the Elemental Wheel is not recommended using the lvl 15 (Ichi) spells for anything beyond magic skill up. Once you attain level 40 and the Ni versions, it becomes a powerful damage and enmity tool in a ninja's arsenal.
Using the wheel you can also increase the chances of landing a Ninjutsu enfeeble, such as Dokumori, Hojo or Jubaku. For example, if you begin the wheel with Hyoton, once you cast Huton to complete the wheel you could then cast Jubaku, which is an ice-based enfeeble, rather than beginning at Hyoton again. Obviously this will break the wheel but makes nice use of the resistance down effect of the final spell.
The elemental wheel can be very effective when properly used- However, the reason you may not see it used often is it requires a large amount of inventory space for ninjutsu tools, and has a very high cost associated with it. e.g. you need 6 inventory spaces for every 99 times you "spin the wheel."