All you have to do to kill the Grim Reaper is get a Ghoul to go in between you and the Grim and wait till the Grim hits the ghoul with his glowing finger and then wile the Grims playing on his fake guitar keep on hitting before his finger starts glowing again
Challenge 21 Complete all other challenges with platinum
Challenges For every 5 Rare bonus books you collect, you unlock a challenge. Collect one in each room for a total of 20 challenges.
Motion picture page Complete challenge 21
Picture book pages Get a platinum ranking in one of the challenges, you get one for every platinum rank you achieve.