LogFAQs > #879448136

Isn't Master Summon missable? Some of the summons you can only get once.

Rest have been covered really well, though I can't stress enough how good Enemy Skill is in this game.

There are two ways to obtain the Master Summon Materia. The first method, and the one that requires the most grinding, is by obtaining all sixteen Summon Materias, mastering them all, and interacting with the red Huge Materia in Bugenhagen’s observatory. The second method, however, requires much less grinding.

After defeating Emerald Weapon, your party will obtain the Earth Harp. Refer to Emerald Eviscerator for more information on downing the powerful foe. Return to Kalm and exchange the Earth Harp with the traveler who gave you the Underwater Materia and he will give you the Master Summon Materia, and you will obtain this trophy.