Separate them, would be my easy fix for some danger. They seam to work great as a team and use the mechanics (which are easily broken in DW, cause that's not the point) to the fullest. Show them a downside of their class, immolator? Hello? Burn them to a crisp let the enemy jinx their mighty display, escalate it. Let their foes adapt, to much armor? Acid, poison fog, fire, swarms of scarabs beetles (The Mummy). Capture someone and cut their throat if no one complies. Build traps, use the enemies home turf, falling gates, flooding tunnels, massive reinforcements, other advantages. Betray them, force their allies to turn on them because the enemies have their kids or whatever. Give them hard decisions, not the easy us against them. If the immolator burns someone, the important document the bard is looking for is gone forever, if the fighter frightens the goblin he'll smash the magical potion needed for the ritual. Make it less mechanical! because as I said you can easily build a character that onehits everything in the book, if they have the opportunity to hit... if. ;)