>>96001011
Even 'non fighter' characters in DF can usually join in a combat with melee weapons and contribute quite a bit simply because most of the advantage an extra fighter adds is just being someone who either guards your flanks or gets around behind the enemy, depending on which side has the numbers. It takes a lot of points dedicated to combat to be as good as two semi-competent guys. The classic GURPS caster tactic is to get one decent buff or debuff effect off and spend the rest of the combat using your staff for area control.
That said, casters thrive against any enemy where they have opportunity to spend a few rounds casting a powerful spell, against anything which is diffuse, or resistant (or immune) to non-magical attacks, vulnerable only to burning (or ice, wind, etc.), hordes of weak enemies (area effect rather than hitting them one at a time), anything which can fly or keep the party at range thanks to terrain (although both barbarian and knight have compulsory ranged weapon skills and can be quite competent with them).