I have a tendency to branch out in different class directions if I have many characters to chose from, that way I have my bases covered. Some enemies can be resistant, or invulnerable for that matter, to different types of attacks and in this way you have an ensured manner of winning.

If it's a small number of characters I tend to use what I call the tank/ranged/healing method. One character with a high level of health/defense and (if possible in the game) attention grabbing techniques, a ranged character (preferably a mage, but an archer will work as well), and a healer whether it be by potions or magic.

As for the single character build rpg I tend to favor the mage class for high damage output and some defensive spells.