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