The PLASTIC TOY(non-violent) Sword is meant to be best against the duper because of its high damage that can defeat duplicates in one hit and doesn't reverse direction when it does hit duplicates. It takes more skill/timing but using the damage power-up is really good for mowing through the duplicates and then hitting the parent ball. With even more timing you can use one of the dash power-ups to hit one of the balls just as the sword gets to the right angle.
You are right, though, there is a considerable amount of RNG that goes into winning, but that is kind of the essence of a game about balls bouncing around. There is RNG, but you can use power-ups to reduce its impact and increase your chances of doing damage and winning.