(From the DFO website):
The Slayer derives his power from the demon Kazan, who dwells within his arm, locking this warrior in a perpetual struggle to keep from being possessed completely. Only the thick, mystical chains shackled to him keep the dark parasite at bay. As a consequence, a Slayer can only wield a weapon in one hand, a disability he has fashioned into a distinct advantage, doing with one arm what most other swordsmen would need two to do. He primarily attacks his enemies with swords and blunt weapons, but can also, when in a dungeon high in demonic energy, call upon the dark power within him to cast magic spells. This dark power also gives the Slayer a particularly high resistance to Shadow energy, but unusually low resistance to Light energy.
Slayers are fairly simple classes to master; their attacks are quick and deal plenty of damage, most of their skills are cheap enough that you won't hurt for MP (excepting that you use skills very often). The basic three-step attack combo is enough for most of the monsters you'll face, and the starting skill Upward Slash makes it easy to chain right in to another three-step. Style points can be an issue with Slayers - the speed of their attacks isn't comparable to that of a Fighter or Gunner and they tend to do more damage, killing targets before they can attain a high hit count.They have "Heavy armor mastery" which improves a slayer because heavy armor and plate armor are the most protective armor there is,but for most characters has an mp cast speed , attack speed, and travel speed deduction, but for slayers , having heavy armor mastery ,do not have this deduction. Nonetheless, Slayers are a great class and are recommended for beginners and veterans alike making them top tier.
Best suited to Heavy Armor.
Uses Short Swords, Great Swords, Katanas, Bludgeons and (in the case of the Blade Master sub-class) Lightsabres.
Blade Master: In spite of the demon's interference, this Slayer continues his vigilant training in swordsmanship to become a Blade Master. He wields the powerful Beam Sword and is able to attack faster than any of the other Slayer subclasses.
Berserker: Yielding to demonic possession, this Slayer taps into an otherworldly rage and becomes a Berserker, capable of a wide range of vicious attacks, including Frenzy Mode, which unleashes furious bloodshed on threatening foes.
Asura: Sacrificing his eyes to gain great power, this Slayer strikes a chilling bargain with the demon Kazan to become an Asura. Though blind, an Asura can master magical attacks and defensive techniques, and sharpen skills that utilize waves and long-range weapons.
Soul Bender: Studying the nature of his demon parasites, this Slayer learns how to better manipulate the dark energies within him and becomes a Soul Bender, whose attack spells are even more deadly, and whose resistance to Shadow energy increases substantially.