If you want to get 100% in one game, not only do you have to give the awesome sphere to the Youth League, you also have to play the game just right. Meaning you are going to have to do the Tobli thing just right, you have to go through the Via Infinito, you have to do all the chocobo stuff, you have to put the Experiment at full power, ect ect. Also, there is a thing at the beginning, when you have control of Rikku and you are running after Leblanc, you have to go off to the left and talk to Yuna in the moogle constume and let her restore your health. If you didn't do that, you're screwed and you have to play through again.

On a brighter note, if you have to play through again, you only have to do enough extra stuff to get 100%, meaning you can skip most of the story, which will make the game much faster your second time through.