Instructions
Recruit a Viera unit if you do not already have one in your party. You can recruit new units by moving your party to a city or castle map location on the world map and selecting "Recruit New Unit" at the bar. New units will also ask to join your party at specific points in the game.
Unlock the Elementalist job class by gaining a White Mage skill and a Fencer skill. You can learn White Mage skills by using staff weapons as a White Mage. You can learn Fencer skills by using epee weapons as a Fencer. Both White Mage and Fencer are available by default for the Viera race.
Change the Viera unit's job class to Elementalist and gain two Elementalist skills. You can learn Elementalist skills by using rods or Elementalist armor.
Unlock the Sniper job class by gaining two Archer skills. Learn these skills by using bow weapons.
Change the Viera unit's job class to Sniper and learn one Sniper skill. Some greatbow weapons teach Sniper skills.
Change the Viera unit's job class to Assassin, which is now unlocked.