The Eidolon battles in Final Fantasy XIII are stupidly hard. These are the battles where you fight against a summon monster in order to gain the ability to summon it in battle.
Now you might be assuming I'm complaining about them being challenging. Except they're not, they're basically just rapidly switching between Paradigms and mashing X. (Or A on the Xbox 360.)
No, instead they're needlessly hard because of the way they're designed.