To forge either a one handed sword with an attack power of 245 or a two handed sword with an attack power of 307 you will need the following items:
One piece of Orihalcon
(this method may work with other metals but the resulting weapon may be weaker).
Five Gold Coins from each of the eight Elemental Spirits (Wisp, Shade, Jinn, etc.).
Five Bottles of Dragon Blood.
Five Bottles of Holy Water.
One Vial of Dragon Breath.
Note: The following instructions will assume that a one handed Orihalcon sword is to be forged and tempered.
Forge your Orihalcon into the HalcSword with an attack power of 75.
Go to the "Temper a Weapon" option.
Start off by tempering your weapon with the Elemental Spirit Gold Coins. The order you temper your weapon with them is extremely important. If done correctly, six of your elemental essence markers will be active. If done in an incorrect order only four will be active.
Apply the Shade Elemental Spirit Coins first. Temper the sword with all five coins or until three Shade Elemental Cards (Mystic Powers on the weapons status screen) appear and have been applied to your weapon. This will raise your attack power by approximately 10 points, raise the sword's Shade Essence Level to 4 and give it the Shade Elemental Essence Marker.
Now that you have finished with Shade, apply the rest of the coins the same way in the following order: Salamander, Jinn, Dryad, Wisp, Aura, Gnome, and Undine. When you have done this the sword will have approximately 140 attack power.
Note: If you want to try changing Plunge Attacks, now is the time to do it. If you try it later it will negatively effect your attack power and waste your materials.
Next, temper the sword with the five bottles of Dragons Blood or until three God of War Cards have appeared. This will raise your swords bonus stats slightly and increase attack power.
Then, temper your sword with the five bottles of Holy Water or until three Cleric Cards have appeared. The first Cleric Card will lower your attack power slightly. Ignore that because the second will increase that attack power moderately and the third card will boost it greatly, so that the swords attack power is around 225.
Finally, to get your sword's attack power up to 245, temper it once with a Vial of Dragon Breath.