Well if you want to finish endgame, you gota lvl them all.
But i personaly went with:
Tressa (main character and also great at filling any role later in game due to hybrid stats) -- spear + bow + wind
Cyrus (seeing enemy weakness is super usefull) -- staff , fire , ice , electic (mostly just to aoe)
Ophilia (healbot) -- staff and light magic (lots of mobs are weak to those)
H'aanit (other monster weakness) -- bow, axe, Lightning, Leghold Trap (usefull for bosses at start)
Later i kicked her out and made Tressa into a hunter and added Olberic (due to his tanking talent).
Setup was like this: Tressa hunter, Cyrus Cleric (as a backup healer or for monsters that were resistant to all 3 elements), Ophilia as dancer (buffs when no healing) and Olberic as Apothecary (or merchant for rest)
I personaly find thief as skillline kinda usless. Armor Corrosive is only realy usefull skill, but you mostly have 1 tank 1 healer 1 melle 1 ranged.
Also later in game you get a chance at Warmaster or Starseer that covers up that weakness.