Okay, this cheat is really good and really easy! It's 100% going to work!
Anyway, first go into the Rougeport Sewers, find a Koopatroll enemy and go into battle. (This is only if you have gotten far enough in the game where mostly Bower's troops are in the sewers.) If there is an enemy besides a Koopatroll, kill it. When there is only one Koopatroll left in battle, it should eventually try to call for back up. After it calls for another Koopatroll, kill the old Koopatroll and you'll get some Star Points. Repeat this pattern untill you have tons of Star Points. When you think you have enough, kill the remaining Koopatroll. DO NOT get more than 100 star points or else it will reset back to you getting around 1 point.
K.#1 --> Calls for back-up --> K.#1 & K.#2 --> Mario strikes K.#1 --> There is only K.#2 left --> Calls for back-up --> K.#2 & K.#3 --> Mario strikes K.#2 --> There is only K.#3 left --> and so on...
I hope this cheat helps!