In order to acquire the Ninja class -- considered to be the strongest class of them all -- you need to go to Eureka (a sidequest area found in the first floor of Sylx Tower) and go all the way through it. When seeing the last teleport (the one who usually takes you to an area with stores), turn all the way left and you will see a floating staff. By touching it you'll fight Scylla, who gives you this class, when defeated.