For sheer capacity to survive, the ideal party centers on a tank. Though in no way mandatory, this is the surest way to exploit the game’s assumptions and advantages.
Players opting for the Hard and Nightmare difficulty settings will still find plenty of challenge in optimizing their party for the later extended battles. So the second step is to consider the inclusion of a healer. A good player may survive on Hard with only a Heal spell or a stack of potions, for the most part, hitting strong and fast to reduce battle times, but tougher moments will prompt a rethink. Again, this plays to the system, so at least one mage is recommended for the breadth and number of abilities they can bring to bear.
If the tank is good at their job then a second mage can be a candidate. There are enough magical disciplines and specializations for further mages to cover entirely different areas – especially damage-dealing and debuffing.
With an archer rogue as the fourth role, you can cover all adventuring needs with a one-melee, three-ranged party build in which your tank keeps the opposition firmly away from their companions. When threat is drawn away from the tank, the distance grants time to spot the break and call a Taunt or focus three-way fire.
Fenris and Isabela provide the companion choice of melee damage dealers. A two-melee party needs to manage threat more carefully but can exploit deathblows and crowd control (Whirlwind, Vendetta) to eliminate injured, hateful enemies as a priority.