Farplane Cup is generally the last cup you can do in the fiend arena. It's arguable if this is hardest or not. This cup contains people from the past and a Super Boss. 2 Main purposes of this cup. Get Tidus, Seymour, and Auron, and defeat Major Numerous.
However both are rather tough to pull off as you need luck on your side to a good extent. You have to actually fight Tidus, Seymour, or Auron to be able to actually recruit them. And even if you managed to beat them you need to win the tournament. Which at times is ruined either by Slimy Boss or Major Numerous (Outer Plane Envoy). Auron can sometimes ruin your day as well since he has Access to Shooting Star. You remember that move? Where he turned fiends into Baseball practice? Yeah he can use that against you.
Slimy Boss can one hit your party members/monsters easily.
Trema: Well you fought him before. You know what that's like.
Major Numerous: Well this should get its own video. Good luck fighting this thing. You're already gonna go WTF is this thing at first sight of it. This guy just loves to kill you off pretty quick if you're not prepared or you're just too slow. But if you want 10 - 30 Iron Duke accessories. It's worth the trouble of fighting it.