The Stone race is a class where formidable bosses can be found. Enemies that belong to this class are weak to Stone Killers. Notable Stone Bosses include Demon Wall, Eggsecutioner Eggstraordinaire, and the Esper Golem. Stone Killer skills are a passive effect when equipped by a unit.
To find the killer gear for the Stone race click here to open the Killer Gear Finder
These trial bosses can be found in the Vortex. The Esper battles can found in the world map. See wiki page for more details.