We will not talk about any role that is ineffective in party, thus we will wholly skip the "killer" and "ranged attacker" one for all but ranger/assassin which have no access to any other. Archer/hunter have few AoE skills, thus they will get a mix. Remember though. Those are the BASIC guidelines. There's no such guide that'll be better than experience(shaiya gold) and cooperation within the party.
Fighter and Warrior in Shaiya:
Fighter/warrior in party usually take role of attacker, along with the chain written above. This class is really straight-forward, so the only thing that may matter is whether you need to intercept a mob running loose, or not. If you are main damage dealer and there is a tank, leave the mob that runs wild to the tank. If you are more of a support in the damage department, or are self-tank, your priority should depend on the skills of the tank. If he's doing his job well, he will regain the aggro, most likely before the mob get's to casters. If he's not as skillful, or didn't notice one running stray, you need to take care of the mobs that go after the weaker casters. We provide Shaiya powerleveling for you.