You can always respec after Fort Joy and it is possible to specialise casters in two or even three magic disciplines.

I like the fights to be challenging and I like to have variety in my party. My party consists of

Ifan, who is a ranger/archer with finesse and huntsman maxed out.
The Beast who is a str and warfare tank using mace and shield, so single handed maxed out
Sebille who is dual wielding with daggers so scoundrel, finesse and dual maxed out
Lohse who uses a staff (though dual wielded wands would be better). She has Hydo, Aero and Necro and can heal, CC and DPS. Very versatile.

I love all of these characters and they all have very powerful moves.