Use 3 times for hints if you need to. There are 3 challenges in the game, 2 with 3 solutions and the last one has 6 solutions. The sixth solution in the last one cannot be done until you complete the story.
Once you have completed the story, stack into the 2 bigger dolls of the Blacksmith Family (dolls you use to forge the crown) and take them to the third challenge area. Use the larger one's ability (breathe fire) then quicky unstack and use the smaller one's ability on the doll you unstacked from. This will cause the fire to explode 'round the place, destroying the ghouls. Don't worry if a ghoul gets you if you have opened a gate, when you retry you just have to head towards the gate, it'll stay open until you complete the challenge.