If you're aiming to make the most powerful group possible, there are some pretty extreme combinations.
For example, against large groups a Hexer can place all 20 hex tokens in a single round (hex someone, plague cloud to a bunch of monsters, viral hex, all combined with the hex doubler), and a Seer Kel Necromancer can follow up right after by simultaneously hitting every single monster in the room (army of the dead, with seer kel's LOS ignoring ability, hits everyone) and exploit those tokens for a +20 damage full room attack. Realistically speaking, nothing survives this. A simple 2-man combo that will wipe out any large monster group halfway through the first turn.
For smaller groups, we have the conjurer. The conjurer can easily keep up 4 images because the RtL AI is too stupid to kill the 3rd and 4th like a good OL would. Meaning every single turn he can attack FIVE times, each with a bonus green die, and the fifth attack getting a +4 damage bonus. This will annihilate any small encounter or boss easily.
And to top everyone off, an Elder Mok bard with the staff of light (pick this up in the 2nd story mission) can heal everybody for 5 health and 4 fatigue every turn, while also providing all other characters with +1 damage to all attacks, making your group functionally immortal, not that it was really necessary since every encounter ends in one turn. It's more about the 4 fatigue every turn which ensures that everybody else never has to rest and can always use their most expensive skills.
To do this, start off your turn by recovering 1 fatigue from concentration, then moving both song counters to Understudy for 2 fatigue, then activating staff of light twice for 4/4 recovery across the group, then moving the two song tokens back to Mending and War for 4 fatigue, and finally healing everyone for 1 more health with mending while recovering a fatigue. The fatigue balances out so this is sustainable indefinitely. Get Wayfarer so you can move 4 spaces per turn for free while doing this.
In our games we generally avoid using any of these combos, as they make it trivially easy, I can't imagine how boring playing with the full set would be