I am close to the end of the game for the fourth time, Duke Castle, went back to finish some quests as suggested by the game itself, now, I tried to this point of the game all possible parties, without entering the tunnel. I feel best party is being Jennell with Sevilin and Nathalie as companions; Shima is getting smarter after collecting some shadowalker specifical items, but I wouldn't change Sevilin with him because of the "wall" that Sevilin represents and that is most useful in more than a circumstance; nor I would change him with Nathalie, whose firebolt at higher levels is unbeatable from middle distance (much better than the razordisk); Then I would not change Jennel for him because of Spirit Charge that I find fantastic when powered by special items, and the beasts, they are not useful to attack but really useful to block enemies and divert their attention and strikes.
In the beginning I discharged Nathalie from the party most of the times because I thought that Jennel was enough effective with magic powers and two warriors were perfect to suit any quest, but I was wrong. The Earthquake in her skill list I also higly recommend.