High level weapons are great for doing damage until you run out of SP. Rather than throw countless potions at it, synthesize a high level (Attack 100, 1000 or better) to a low level one found in the Forest of Embracing on Normal (the lower the better).

The resulting synthesis will average the SP consumption, and with some luck, allow you to retain a measure of potency of your high level weapon (about 70% of it at worse).