Bring Rigard, Sabina, Gunnar, Balthazar, John, axes, arrows, small and medium mana drinks.
Try to keep Gunnars buff active all the time when facing the bosses, heal your team, fire an arrow from time to time and keep the bosses calm with John and axes.
John should be used right before a huge combo/cascade will go off and if the bosses have only little mana. Axes should be used to weaken their specials and arrows to blind their normal attacks.
Focus on the boss which will probably cast first or the bigger threat.
If your team is about to die and you are out of mana drinks to heal it, throw your arrows and axes, which possibly will finish at least one of the bosses.
If one boss is dead, try to match tiles into the emptiness to charge yourself faster while charging the opponent slower.
Good luck for the darts… 
Later on those rare quests aren’t much of a challenge any more.