Final Fantasy V

Monk is one of the best damage dealing classes in the game, and once mastered, the base class gains the Counter skill TEMPORARY-DASH-CHARACTER which is damn useful.

Learn L5 Death in the Ancient Library. I think Page 128 uses it and to learn it the blue mage needs to get hit by it (and die) then get revived before the battle ends.

Learn Doom Claw when escaping from the fire castle after the Fire Ship. The boss uses it and it paralyzes the target and it brings their HP into the single digits range. It even works on some bosses!

When you reach the first castle in FF5, go into its basement. Its filled with statues who are vulnerable to L5 Death and give you the most AP at that point in the game. Great time to grind classes!

Mastering Hunter gives you RapidFire which is 4 attacks at half damage and perfect accuracy.

Near the end of the game in the town outside the forest you get the choice to get a Brave Sword (gets stronger the more fights you win) and the Chicken Knife (gets stronger as you run away more). Take the Chicken Knife, the Brave Sword gets weaker when you run away but the Chicken Knife doesn't and its damage calculation takes Speed and Strength into account. Its the best weapon of the game!

Bare/Freelancer get the passive abilities of all classes you've mastered with few exceptions (like AutoBerserk) and the highest base stats of any mastered classes. So if you master Monk and Knight, the Bare class gets the best stats out of both classes and all passive abilities the class has, like MaxHP+30%.

Berserkers are broken. In a good way. Nearly any fight you're having trouble with can be won by throwing caution into the wind.

If you don't want to grind for cash, try to assign a type of armor to each of your characters. By this, I mean one would always get classes with heavy armor, one with medium/light, etc. and have that character stick to classes that use that armor. That way when they change classes they can keep using a lot of junk they just had equipped and you won't need to spend lots of money by new gear.

Blue Magic is very useful if you get the spells as early as you can. Also bosses are usually susceptible to a status effect or two. If you're in a tough spot, there's no harm in giving L.5 Doom or Odin a try...

A thief will find the secret passages for you. Completely mastering a class will provide a big boost for your character, depending on the class.

It's better to master jobs than to half-ass them.(Counterpoint: The only time mastering classes comes in handy is if you go No-class for important fights. In that case, you'll get the stat benefits from your best mastered class, and all the inherent abilities of mastered classe -- ince Ninjas automatically have Two-Weapon, a No-class character with Ninja mastered will also automatically have Two-Weapon. For your endgame, the objective best classes to have mastered are Monk (highest STR and STA modifier), Thief (highest speed modifier) and Summoner (highest magic modifier). Aside from those three, figure out which classes have inherent abilities you like and figure out what 2 (or 3, with Mimic) equippable abilities you'll want to use as No-class. Even if you're not trying to min-max, Monk is a good class to master for your fighters due to high HP and inherent Counter-attack, and the Archer's X-Attack is awesome as an equipped skill. Black Magic is also kinda weak in FF5, too. It gets a little overshadowed on damage by summoning spells, and its not nearly as versatile as summoning or time magic either.)

I turn one set of character in physical attackers and one into mages. Granted, for some jobs like Thief/Monk, you'll want to master the class on everyone, but the two ultimate builds are a magic build and a physical attack build.

Two important notes for boss battles: 1. You can often nail them with a debilitating status effect. 2. Each boss usually has a glaring weakness to a job or two.