Hello,
I will submit this suggestion for you. I'm sorry that you've gotten this mission and haven't been able to make progress on it. In the meantime, I can offer a simple solution for you that will help you easily beat it.

Simply remove the ninjas from your team and have a friend beat you into a lower rank. Then put your ninjas back on and beat someone else to get a higher rank. I would then suggest you don't immediately take the highest rank possible, that way you can get a higher rank later for this event when you need it.

Thanks and good luck!