Credit goes to Primaclaire from
http://www.playstationtrophies.org/There is a bug that you can exploit in the v1.0 of the Japanese version (I'm not sure if it works with the Asian version). Simply store your ores (the materials needed to upgrade equipments) to the storage and if you try to upgrade weapons, it will not consume any materials at all.
Even without the bug exploit, you can get this easier by simply upgrading a certain stat on a weapon, and then switch to another stat so you will only spend 2 materials and have a high chance of success.