I haven't ever come close to completing the entire sphere grid, although my brother has come quite a ways, he has activated every node with three of the characters (Tidus, Yuna, and Kimahri), and he's changed tons of the nodes using spheres (example: clearing a HP +200 node, and replacing it with an HP +300 using an HP Sphere, and doing the same for the other stats, including Luck).
Took him quite a bit of time to do just that alone, but he has three of the characters having over 96000 HP, so it's proved worth it, he can beat Nemesis consistently.
Took him quite a bit of time to do just that alone, but he has three of the characters having over 96000 HP, so it's proved worth it, he can beat Nemesis consistently.