So for anyone who doesn't want to grind levels forever, 5-11 is a good place to go. I know it is late in the game, but using only as many turtles as you want, and figuring out how many monkeys you need you can get money and levels easily.
For those who are having problems killing the last ninja on level 14, get your meteor spell up to max, and make sure you have the skill that lets you get 20 mp when you place a temple. Make sure the round is going slowly, and when he appears, hit him with 5 meteors. If you have been able to build up catapults near the entrance, then he will die with just this. If you don't have these, then quickly build 5-10 temples using the shift key. This should give you enough mp to use meteor once or twice, which should kill him.
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