really? What level are your characters at? mine were at 60 and I got to level 50 no problem. You don;t have to fight the Tonberries that you see, you can actually just avoid them. Put grids that add to your HP, have High Magic defense and give you the abilities of a white mage. Better still, have an Alchemist, they're better. AVOID THE ELDER DRAKES on level 19... They killed me soooo hard I never even saw it coming. He jumps back and forth so as soon as he jumps backwards, pass him. If there's no way to pass him, go back to the save point. It remembers what level you were on last so, just go back to Level 19 and avoid him. The levels change all the time so he could be in an area that's easier to pass.

Equip stuff that will bring your HP up like a Diamond Bracer and keep spheres with High HP, like Dark Knight or Warrior.

Good Luck. the last sphere you're looking for is on level 20. the Infinity Level (100) is just to get a Completion in Chapter 5.