The main purpose of a shield is to just improve your defense (and maybe also get extra benefits for specific shields). In the majority of cases, the best defense is a good offense. So wielding two weapons will kill off enemies faster than they can kill you.
Shields then serve to reduce damage when damage is inevitable. And to that end, it's best on defensively minded classes that will be protecting all your allies who do have two weapons. These would be the Knight, and in the DS/iOS remake the Viking. All mention of Viking beyond this point refers to the remakes, as the original Vikings did not get a special skill.
The Knight's Cover ability lets it take damage in place of weak allies. Meanwhile, in the remakes, the Viking gains the Provoke ability which expends its turn to attract attacks. Since this means they'll be the ones taking damage, equipping them with shields is usually a good idea so that they can take more damage.
Vikings in particular, an actual common strategy is dual wielding shields. Since Provoke takes up your turn, you won't be attacking. Two shields gives you a whole lot more defense, your axes and hammers tend to miss anyway and you don't have nearly as strong an offense as the more attack-geared classes. So, you could have a powerful party of attackers that can safely attack while one dual-shielded Viking takes all the hits. This is especially helpful for those bosses who throw in an extra physical attack on their turns.
More information about the job guide that you can read. The following information below here will answer your general questions about job levels.
Q: What is Job Level?
A: Job Level (Job LV) is a renewed idea for the DS version. Each character can
have, at most, 23 jobs, and every Job comes with a Job LV showing how
proficient that specific character is at that Job. Maximum Job LV is 99.
Q: What does Job LV do?
A: The more you train as that Job, the higher the level will become, and the
better your character will perform! Here is EVERYTHING that Job LV affects:
- Number of hits when attacking.
- Chance of executing a Critical Hit.
- Physical damage.
- Black Magic damage.
- White Magic recovery.
- Summon damage.
- Attack Power of bare-handed Monks and Black Belts.
- It also enhances the special abilities of certain Jobs.
- In one of the side quests, a Job Mastery Item will be unlocked when a
character reaches Job LV 99.
Q: What is NOT affected by Job LV?
A: The characters' overall stats are not affected by Job LV. Strength, Agility,
Vitality, Intellect, and Mind.
Two characters with SAME Job and Character LV, but different Job level, will
have the same stats. However, their performance may differ.
Q: Speaking of the Job Mastery Items, can I get more than one?
A: There is ONE item per Job, and you can only get each of them once. In the
end you can only have 23 of them.
Q: How does the Job LV increase?
A: Every character has a hidden stat called Job Points (JP) that dictates their
Job LV growth. When JP is accumulated to 99, Job level goes up by 1. Each
action (except: Front, Back, Escape, and Flee) will contribute a fixed amount
of job points to the character.
Best Answer
Any combination will work. It's been a while since I went through the 'small' part, but even then you should just buy some basic dmg spells, and temp switch other classes to lowbie casters. In fact, for 99% of the game, black magic is weak, and vastly overshadowed by any melee: while the 9,999 dmg cap is visually reported, my knight and monk have hit for 59,999 (at lvl 98/job level 99) (per achievement page, so this is probably the hard internal cap). There is also certainly no need for a dragoon. If a boss is hard, keep levelling up, then come back.
Simply choosing your favorite classes is fine (a white mage, for heals, though, is recommended).