Attempting to complete this challenge alone can seem impossible and cause unnecessary stress when attempting it.
Taking in a party to complete the challenge with you can make it easier to plow through the enemies.
Have one person enter a room and lead the youkai towards the others (called a "lure") while the others use their blast on the approaching youkai. This will eliminate the youkai in one shot.
When fighting the Nurikabe on the second floor, have one person grab it's attention while the other members of the group attack its sides. It will ignore the people on its sides and keep its attention focused on the "lure".