The Machine race (aka Machina) is a class where formidable bosses can be found. Enemies that belong to this class are weak to Machine Killers. Notable Machine Bosses include Aigaion, Omega, and the Esper Alexander. Machine Killer skills are a passive effect when equipped by a unit.
To find the killer gear for the Machine 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.