Activate each Class you have access too.
Always start with the lowest level quest in the area, whether side quest or main quest. Let the level of the quest determine order.
Progress a Class 5 levels, doing this for each Class. Keeping them all even.
This allows you to play main story quests and side quests with all of your Classes instead of just one.
They all benefit from the experience and you end up right at if not slightly over each quests requirement.