the easy way to get s rank is
go to mission go to training missions go to advanced fight training all you have to do is kill 47/47 for s rank and kill the the big dragon under 30 seconds just push the X button 2 times you will do a special attack.
Sidenote-hire a wingman to make it alot easy any dragon for 10,000 to 11,000 will work.
I think we have 11 days left on this challenge.
you don't not need a max out dragon for this solution all you need is money to buy to hire the wingman.