FINAL FANTASY VII
32 Achievements
40-50h
Transcendant Convoker
Obtain the Master Summon Materia
13.5%
How to unlock the Transcendant Convoker achievement in FINAL FANTASY VII - Definitive Guide
Method 1: Master all 16 summons in the game.
This is likely the quickest way, but there are pitfalls.
5 of the Summon Materia are missable (Ifrit, Ramuh, Bahamut, NEO Bahamut and Phoenix). So if you're at a point in the game where you've missed one or more of these, you're going to either have to start over, or use method 2.
The fastest way to master materia is covered in this video (for which I claim no credit but it's the one I used)
Just equip the materia you want to master, and go the location in the video and get grinding.
Method 2: Defeat Emerald Weapon and take the Earth Harp to the traveller in Kalm Village, he will then give you a Master Summon, a Master Command and a Master Magic Materia.
There are several guides (including one from yours truly) on this site to defeat Emerald Weapon
Good Luck
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 (though do note that Ramuh is missable), 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.
The Earth Harp can be traded with the Kalm traveller for the Master Summon Materia.
But there are two points: you must have red Huge materia and all summon materia (some can be easily missed).
After you pump all the materia into the master, you will be given this master materia instead. The same can be done with yellow (you only need to download certain ones) and green. All this is done in Cosmo Canyon, in the planetarium of grandfather Red the 13th.
The method is good so that you can then go on to super-bosses with master materia, and not get it after destroying one of them.