The Bug race (aka Insect) is a class where formidable bosses can be found. Enemies that belong to this class are weak to Bug Killers. Notable Bug Bosses include Nozuchi, Sand Worm, and the Chamber of Arms' Shaula. Bug Killer skills are a passive effect when equipped by a unit.
To find the killer gear for the Bug 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.