I tend to fight defensively unless I clearly have the advantage. When I encounter a battle where I'm outmatched I go back a save and overlevel. A lot.

Early in game I tend to favor a mix of classes that I'm learning but with nongeneric characters I tend to keep their default skillset as secondary. Like say Agrias as a White Mage with Equip Sword and Holy Sword. I swap them back to default classes in boss fights.

Late game I tend to have level 99 characters so I equip Math Skills on almost every character and use calculations that affect multiples of 3 to heal and other multiples for attacking foes. Often I wipe out foes before they reach meelee range.