Ni no Kuni: Wrath of the White Witch

About halfway through the game (when you reach Yule or so), the Solosseum Series will open at the Temple of Trials. Speak to the assistant at the back, and you will be able to choose from a series of fights of increasing difficulty. By the time you open it, you should be able to handle up to Rank D easily (the second fight). When you win the first fight, you get Solosseum Slugger. When you win the 6th and final fight for S rank, you get Solosseum Supremo. If you save this for the end, the fights are not very difficult, but if not, the final fight in S rank can be tricky. It is a fight vs gold versions of your own 3 starting Familiars (Mitey, Drongo, Hurly) - Mitey will spam his AoE and Hurly will spam rock throws, which can hurt a LOT. One helpful tactic is to use a tank Familiar and use Yoo Hoo or Maddening Melody to draw them all to you so your teammates can do some damage. You also can have Swaine use Scattershot on Hurly to have him do some damage to his teammates.

As a marker, I did this with my 3 people at level 90, and had no issues.