Answer:
There are only 2 ways to get powerful weapons; scrolling and buying scrolled weapons.
There are 5 types of scrolls which you can use:
10% scrolls - work 10% of the time. Add 5 attack (not sure about magic attack)
30% scrolls - work 30% of the time. Same as 10% scrolls but can "blow up" the item 50% of the fail rate.
60% scrolls - work 60% of the time. Add 2 attack (not sure about magic attack)
70% scrolls - work 70% of the time. Same as 60% scrolls but can "blow up" the item 50% of the fail rate.
100% scrolls - work 100% of the time. Add 1 attack.