i know i realised when i spent about 100,000 powering up laser weapons thinking i had missed my chance to get things such as mythril sword but thats why i'm going through the game again so i don't waste my synthisis matreials.
i think that the best thing to do is to use whatever talent is needed on what your synthisising to duplicate the effect a couple of times then synthisise it to the weapon the use smithing which duplicates the effect again so if you where to use alchemy (Can't remember) on an orichihalcums it'll duplicate the effect but cost quite abit
its effect is +500atk you use whatever the talent is on it it should get doubled if you give it the time to makeit should look like this
+500 atk
+500 atk
then synthisise it to the weapon
then use smithing on the weapon to your heats content
i know that this can happen a total of 8 timee so when you have 8 500atk upsyou have just become extremely powerful but if that doesn't work reset the game and use alchemy or compounding on the orichihalcums again until the orichihalcums has the effect of technicly +4,000atk then synthisise it to your wepon of choice with a mythril sword preferably that be +4,800atk but then if you could be bothered to level up so much that you could go through the Srank loner battle and get them scroll that were +1,000atk i belive that using writing on it should duplicate the effect that 8 times with fayts best weapon he gets an extra 10,000akt (9,999 if it can't go any higher)
although with sophia her best weapon gives her +2,600 atk on its own
so for some reason she's stronger then fayt.
but be warned synthisising somthing you done up that much will cost a hell of alot of money somewhere in the region 100,000Fol every time gauge.
there you have it with an attack like that the last hit of fayts dimension door should be 99,999 every time.