By far the most time-consuming achievement in both FFX and FFX-2. To be clear, the achievement requires you to activate every single node on the Sphere Grid on all 7 characters. Yes, this includes unlocking every single lock and filling every single empty node with a stat sphere to activate it. The Expert Sphere Grid is far easier to accomplish this with. It has significantly less locks and total nodes than the Standard one.
Late game stat-maxing is a lengthy and multi-step process detailed in multiple places over the internet, so I will just explain things unique to this achievement specifically.
1) You will want Triple AP weapons from One-Eye for all 7 characters for the Don Tonberry AP trick. You can customize Overdrive->AP on all 7 with items gotten from capturing for the Monster Arena, but only enough for 3 Triple Overdrives. If you want Triple Overdrive for all 7, you will have to farm gil to bribe Sand Worms for 900000 gil but it's not really worth it. The quicker method in my opinion is to use the Underdog's Secrets from Rin for completing the Al Bhed language to customize Double Overdrive instead. Then customize SOS Overdrive onto the character who is taking 99999 from Karma. Just have him get continuously killed and revived as you feed AP to the other 6 characters and he should reach 99S.Lvl's with them.
2) You really only need about 85 Luck to beat all the Dark Aeons and Penance if you play the fights smart. People often want more, and it does make it a little easier. But the problem is that for every Luck Sphere you put in the grid for your superboss killing party, ALL SEVEN CHARACTERS need to activate it. Meaning farming an ungodly amount of Fortune Spheres from Earth Eater. Trust me, Clear Sphere every Luck node that came default with less than a +4 bonus, which is 6 out of 7 on the Expert Grid. Then add 16 Luck Nodes. That will mean 17 +4 Nodes total, +68 Luck, sending all your characters to 85-87 Luck. 17 times 7 is 119, meaning at least 60 Earth Eater kills for Fortune Spheres, significantly more due to Dark Matter drops. I do not recommend using Clear Sphere's for any other stats. Just activate those nodes, there's plenty of space to max everything you need and then some even on the Expert Grid.
3) When you're happy with all the stats you have, the best "filler" sphere for the grid is Magic Defense Sphere's or Evasion Sphere's. Some people want to Break the HP Limit and fill with HP sphere's, but 9999 HP is sufficient to beat the hardest bosses. Magic Defense/Evasion are the best because One-Eye/Pteryx are the simplest to kill quickly and efficiently in two hits without retaliation, and the Mana/Speed Sphere's to activate them are just as easy to get as Power/Ability Spheres from Kottos with the distill skills and items.
Be patient. This is roughly about as grind-heavy as Treasure Hunter from XIII, but it's not as bad as 10000 kills in VIII and IX! I would wish you good luck but then you would just need to kill more Earth Eaters.