I decided to do some scrounging about looking for upgrade guides and things and there was not much in English really, but I found a Japanese WIKI with some really nice info, such as amount of EXP needed to reach max lvl for each weapon. This helped me a lot using the 36x Sturdy bone method to reach 3x EXP multiplier, since I could work out exactly how much of what I needed.
36x Sturdy Bone = 3x EXP Multiplier.
At the time, I could only buy the 840gil items, I think they were 774(?) EXP.
I upgraded Axis blade, so I divided 66,750 / 774, and bought the correct amount of items. Dumped them all on the weapon and BAM, 26* lvl axis blade in less than 1 minute.
I thought I would translate it. Hope its OK, I know it's not gamefaqs here. I only did Lightnings swords but if it's ok, I'll do the other characters if there is interest. All names are from the english version.
Format goes:
Weapon name: Attack @ highest lvl / Magic @ highest lvl / Ability / Item to change weapon / Amount of XP needed to reach max lvl.
Lightning:
Blazefire Sabre > Flamberge > Omega Weapon.
Blazefire Sabre: 115 / 115 / --- / Pervoskite / 24,600.
Flamberge: 323 / 323 / --- / Trapezohedron / 445,950
Omega Weapon: 620 / 620 / ATB+1 / --- / 1,296,897.
Edged Carbine > Razor Carbine > Omega Weapon.
Edged Carbine: 83 / 170 / --- / Pervoskite / 33,000.
Razor Carbine: 190 / 448 / --- / Trapezohedron / 437,550.
Omega Weapon: 210 / 723 / ATB+1 / --- / 1,296,897.
Gladius > Helter Skelter > Omega Weapon.
Gladius: 175 / 88 / --- / Uraninite / 33,000.
Helter Skelter: 448 / 190 / --- / Trapezohedron / 332,560.
Omega Weapon: 723 / 210 / ATB+1 / --- / 1,380,087.
Axis Blade > Enkindler > Omega Weapon.
Axis Blade: 48 / 48 / ATB charge on hit / Adamantite / 66,750.
Enkindler: 133 / 133 / ATB charge on hit / Trapezohedron / 280,830.
Omega Weapon: 315 / 315 / ATB charge on hit + ATB+1 / --- / 1,419,867.
Lifesaber > Peacemaker > Omega Weapon.
Lifesaber: 88 / 88 / Improved Raise / Scarletite / 70,850.
Peacemaker: 225 / 225 / Imroved Raise / Traezohedron / 236,350.
Omega Weapon: 520 / 520 / Improved Raise + ATB+1 / --- / 1,460,247.
Lionheart > Ultimate Weapon > Omega Weapon.
Lionheart: 68 / 68 / Early Stagger / Scarletite / 63,060.
Ultimate Weapon: 213 / 213 / Early Stagger / Trapezohedron / 221,900.
Omega Weapon: 508 / 508 / Early Stagger + ATB+1 / --- / 1,482,487.
Hauteclaire > Durandal > Omega Weapon.
Hauteclaire: 138 / 138 / Stagger Lock / Uraninite / 54,800.
Durandal: 378 / 378 / Stagger Lock / Trapezohedron / 332,560.
Omega Weapon: 711 / 711 / Stagger Lock + ATB+1 / 1,380,087.
Organyx > Apocalypse > Omega Weapon.
Organyx: 187 / 187 / Leadenstrike / Cobaltite / 20,100.
Apocalypse: 410 / 410 / Ironstrike / Trapezohedron / 233,220.
Omega Weapon: 750 / 750 / Ironstrike + ATB+1 / --- / 1,514,127.