Toady will never give you any hints when it comes to the Legendary Weapons, but you can make them cooler (like you did with the Desert Seeker II). If you fuse the weapon with another one at or above a certain level it ranks up.
Check gamfaqs for the full list
Desert Seeker II = Desert Seeker + lvl 11 weapon or higher
Desert Seeker III = Desert Seeker II + lvl 18 weapon or higher
Earthshaker = Desert Seeker III + lvl 26 weapon or higher
Once you have the Earthshaker don't bother fusing it anymore. Just use it in battle until it the stats are maxed.