Minstrel / Armamentalist: This party member is well balanced in all areas and excels at supportive abilities such using egg on (assuming if this is the main character's vocation, since egg can only be learned by them) to increase tension and the 4 types of forces to increase resistance and offensive capabilities against certain enemies or bosses. This member also should invest some skill points in aquisitiveness to learn half-inch as other members can fight while stealing items from enemies.
Mage / Sage: This party member is very useful in offensive and a little bit of supportive magic (like oomph) in combat. Their low hp and defense can be fixed by investing points in vocations that increase those stats. They tend to have the highest mp of the group. It is recommended to spend skill points in spellcraft and enlightenment since when the points reach to 100 (in spellcraft), magical might increases by 100, and when skill points reach 100 in enlightenment, mp consumption drops by 25%. Note: Do not sell the Dragon robe; it is needed it to make the xenlon robe which is very helpful to get as it provides 42 points in magical might.
Warrior / Gladiator: This member of the team excels at defense and offense capabilities (in other words is the tank of the party). They also have through the roof hp which most of the time makes them last longer in battle than other party members. Give them the majority of yggdrasil leaves and dew because of that. Gladiators tend to have less defense than warriors so be careful of that. Also, gladiators normally can't equip shields, but this is a minor issue since if 100 skill points are spent on the shield skill, any vocation can use one. Note: This party member should equip a Meteorite bracer (increases agility by 100) for a better chance to attack before the enemy.
Priest: This party member specializes in magical mending. It is helpful to invest 100 skill points in faith, since this increases magical mending by 100. The priest should also invest 100 skill points in enlightenment to learn right as rain and consume 25% less mp. Priests will run low on mp quicker than mages, if equipped with anything other than a wand. Note: It is critical to keep this party member alive since this is usually the primary source of hp recovery.