Final Fantasy XII is one of the first games in the Final Fantasy series that actively encourages the use of a standard MMORPG party setup which includes a tank, a healer and supporting characters. This can be modified in subsequent releases of the game (such as the International Zodiac Job System version or the Zodiac Age version) where jobs become hybrids, but for the most part, new players should setup their characters as follows:
One character should be equipped with strong armor and a shield. Their armor should nearly always be Heavy Armor except in specific cases where strategy calls for the use of specific pieces of equipment and they should be the only character equipped with a Shield. No other characters should be equipped with a Shield (again, with the exception of specific boss fights that call for specialized equipment).
There are some boss fights in the game where the boss will ignore evasion. In these cases, even the tank should switch from a one-handed weapon and shield combination to a more powerful weapon, such as a two-handed greatsword or a spear.
It is impossible in the early game to force enemies to focus their attacks on the tank specifically, but once your party reaches the Eruyt Village, you can purchase a spell called Decoy. Decoy can be cast on a party member to have enemies focus their attacks on that character (which should be the tank). The effect, once cast on a party member, is listed as a “lure”.
Note: Decoy was setup or programmed poorly within Final Fantasy XII. There is a chance that the casting of Decoy will “miss” the target player as the target’s Magick resistance and the caster’s Magick stats will affect its ability to hit, unlike every other beneficial status effect in the game (Protect, Shell, Haste, etc.). For some reason Decoy operates like a negative status effect which means that it could fail (sometimes multiple times in succession) which can cause major problems for your party.
All parties will require one character focused primarily on healing spells. This is very similar to other Final Fantasy games, but healers in Final Fantasy XII should focus on a hybrid approach. Conserving MP as a healer is difficult, so all healers should focus on obtaining three Quickenings as fast as possible, picking up the Headsman, Martyr, Inquisitor and Warmage Licenses as soon as possible, but this means that they also need to be attacking as a means to restore MP.
Certain sections of the game and certain boss fights will require that you spread the healing out over multiple party members, again, to help with the conservation of MP.
Support Characters in Final Fantasy XII represents characters that are focused on dealing high amounts of damage and those that are focused on supportive enhancing buffs like Protect, Shell, Haste, Bravery, Faith and Bubble. Again, all of your party members should be splitting these duties, but your support character will not be tanking and not be healing (primarily at least) which allows them to play a hybrid role supporting all functions required.
Buff spells (like those listed above) are incredibly important in Final Fantasy XII. All major boss fights are made much easier by ensuring that your party members have these spells activated for the duration of the fight, much more so than in previous Final Fantasy titles.
By no means are the above three job classifications required. There are a ton of strategies that can be used, some which are much more effective than those listed above, but this strategy should help newcomers to the game get a feel for the battle system of Final Fantasy XII.