I have discovered the following party build that has withstood hordes of rampaging enemies on nightmare difficulty, with no potions, and no health, that can be established at beginning levels. Yes, I said no health.
The most recent test was fighting three wisps, one pure wisp, and three bog-swamp-rhino-bore-rampaging-things, at once, on level 9. I remember Cassandra was bulldozed by those two creatures 30 feet from my party with very little health, yet we survived when I ordered my team to disengage and stand around her, provide protection while she recovered. I think that battle end inside a log cabin, with a battle time of 15 minutes?
My party is two spirit-mages and two shield-warriors with less than 100 armor rating. The shield-warriors are not two-handed warriors, but shield-and-sword warriors.
Here is my theory on why this party is so successful:
The two shield-warriors are specialized in the weapon and shield perk tree and the a little bit of perks in the guard perk tree. All of the perks in those trees help the warriors take tremendous punishment. They also randomly generate guard protection, but I am not sure where that comes from.
The two spirit-mages specialize in all of the spirit support spells, three of the biggest are barrier, dispel, and revival. Barrier protects your two warriors from all damage for a limited time. Dispel removes magical traps created by enemy mages. Revival brings your whole team back to life minus the caster.
The mutual team work occurs when the two shield-warriors taunt the enemy and draw fire from the mages. The mages provide magical defense for the two warriors, and occasionally themselves. Having two warriors doubles the defense against enemies flanking your two mages. Having two mages cuts the defensive spell cool downs in half.
I said no health required. Having two mages with barrier, one can almost spam continuous magical defense on either the two warriors or all four party members. Because the warriors also randomly generate protective guard on top of their health, this combined with the nearly continuous generation of protective barriers decreases the window of damage opportunity to once every 10 to 20 hits. Using my two warriors and two mages party, I was able to restore, dispel, and revive my party through the hairiest battles I would of thought impossible to survive when I first started playing, after I had used all of my potions and everyone's health was at their lowest.
I have also learned additional tactics with this party. The mind blast can interrupt any enemies attack. This is very useful for pausing the lesser-terror-stick-tree-like-screaming-popping-out-of-ground-creatures from stunning your team almost indefinitely. Remember, you have two spirit-mages to spam the mind blast spell. I found myself actually moving the mages closer to, but behind the warriors at all times, so they can utilize the mind blast spell. This turns into an elegant dance of the party, where the warriors attack the current highest property enemy of your choice, and the mages dance around the warriors. Any enemy that tries to target the mages, either becomes taunted by the warriors, or they are mind blasted. It helps to have at least one additional crowd control spell on the mages, such as freeze or shock, but optional. I found battle proximity for the mages is matter of preference for the player. The revival spell is the most devastating weapon in the party. Once the enemy has thrown everything at the party to near exhaustion to kill all but one of the mages, that mage will revive the whole party back, with additional protection from the upgraded revival spell. If the party dies again, you can cast revival a second time with the other mage. This party is a living nightmare for the enemy.
One issue resolved : The barrier spell is double casted automatically by both mages, which waste the opportunity to cast them in chains. I resolved this problem by making sure I was in selection of one of the two mages in the tactical view, which prevents the game from auto-casting the spell, and allowing the other mage to cast the barrier spell automatically.
Disclaimer : The full benefit of this party build requires full use of the tactical view. The smallest window between chain casting barrier with two mages was less than three seconds, but keep in mind that the barrier spell is only one out of many different defensive tools at your disposal, such as: all of the warriors crowd control attacks, the randomly generated warrior guard, the strategic positioning of your team mates to minimize damage from incoming spells and arrows (such as attacking archers first and moving the mages behind the archers), the mind blast spell, and the revival spell.
Additional Disclaimer : This party is not invincible, but it is a die hard party. Because of the 3 second window between chaining barrier spells, there is a possibility that one attack out of ten will kill one of your team mates with no health.
I challenge anyone to try my suggested party build and test if it works or not to what I have written.