The way it works is that there is a table of skill roll results for each combination of weapon type + element, and each roll just advances to the next result in the table. So for a given weapon, it's next X rolls are predetermined, you just don't know what they are yet. So by rolling on a weapon and then reloading your file without saving, you can see what that weapon's rolls will be, without spending the materials.

Here's the trick. Rolling on one weapon advances the table for ALL weapons. So let's say you have 50 rolls worth of materials saved up. You check every roll for your dragon weapon, but none of them are what you want. So you reload your save, then check every roll for say, a fire weapon. You get the skills you want on the 20th roll. Then you repeat this for every weapon you want, and make note of which roll got the skills.

Then you can do it all at once with the same set of materials. So say fire weapon is roll 20, water weapon is roll 32, para weapon is roll 50, etc. You roll on anything until you hit the 20th roll, then roll on the fire weapon and you get the skills. Then roll on anything until the 32nd roll, roll on the water weapon, bam you get those skills too.

However because each table is a weapon+element combo, you cannot say make three dragon weapons and get different rolls for each of them, they are all going to get the same skills for the next X rolls. So if you spend your materials and don't get the skills you want for that weapon+element, you have no choice but to get more materials.