There's a quest in Rendor for the Fierce Warrior book. With it, you can fight stronger versions of story bosses (and get orbs) and do all the ordeal gates you previously cleared in the story. For the ordeal gates there is a once per week limit on how often you can go. You can still go with another player who hasn't cleared them for the week, so if you know other players to go grind with, you can do them more than once per week.
It's a good idea to bring a thief, for getting better and more loot. But generally a party that is strong enough to clear the ordeals as quickly as possible is all you need. Some of the ordeals required certain skills (party member combinations) to defeat at lower levels.
It's a good idea to bring a sage for the zero baptism skill that removes buffs and tension from opponents. The Cedra Dragon might require two healers, for the Diva you want sleep and some other affliction resistances.
It becomes a non issue with support characters above level 90 since they can defeat most enemies before they get dangerous.