From what I can remember, Illusionists are mages that cast spells that can reach all enemies or allies on the field. They have a low defence, so it's a good idea to keep them holed up in a corner of...
well it depends ill tell you the human jobs hunter: you need 2 archer action abilities paladin/fighter:you need 2 soldier action abilities blue mage:1 black mage a ability and 1 white mage a ability...
By mastering 1 White Mage A-Ability and 1 Fencer A-Ability as a Viera. Elementalist can also lead to Summoner and Assassin, Which is my personal favorite.