Ok, I will try to give a brief and vague explanation of how this works. The 122333 Challenge is not as simple as you may think. There's allot more to this challenge than just accessorizing. It's a totally different style of gameplay from a normal game!
You must apply many different strategies in order to survive and keep your party from leveling up. The Firefly will keep your party from leveling up, but you can't get the Firefly until Stillshrine of Miriam. You can get it there and get more for your party from rare Slaven drops. (Mosphoran Highwaste) That means you are forced to flee, duck and dodge up until then. The only things you can really fight are the Hunts, "mandatory" enemies and Boss battles to progress. Otherwise you will level up. This means that Gil is scarce and License Points are limited unless you chain Ba'Gamman in Lhusa Mines. In other words it's not easy at all unless you understand and have mastered all the ins and outs of how Final Fantasy XII works.
The Pheasant Netsuke improves potency of restorative items such as potions. Can be bought in Old Arcades and rare steal from Mu. (The Feywood) Your best friend in 122333 is the Fomalhaut. Indigo Pendant and Nihopalaoa also play a critical role in the 122333 challenge.
I played a Fran Solo 122333 until after Laviathan. I did this because I want it to be a "True" Fran Solo and keep everyone else in my party unleveled and at base level. The only way I could accomplish this is to play 122333 until after Laviathan. It was one of the most difficult and frustrating things I've ever tried to do. You should try killing Mimic Queen with only using Fran at Level 2! (It sucks) Needless to say it's over and I can now start having fun in my Fran Solo game. My party will eventually look like this: 1,2,99,3,3,3.
I have allot of respect for anyone who takes on the 122333 Challenge, Solo, or especially LLGS. (Low Level Game Solo)
Best of luck to all who attempt these challenging feats!