Geomancers are basically inflexible Black Mages with infinite MP. Which attacks they perform is based on the terrain or dungeon you're fighting in, but unlike later games it is not random. The only random element is that if the Terrain ability misses all targets, the ability will Backfire, reducing the Geomancer's HP by 1/4 of its maximum value. As a result, a Geomancer's effectiveness in a particular dungeon is largely based on which Terrain ability is available in that dungeon. Terrain abilities for dungeons are listed on the dungeon pages. On the overworld Map, each type of terrain has a different ability.