Best place to hunt guardian 20?

Ok i am going to invest in a 24 hour drop card for a few things, one of those being guardian 20. What i would like to know from those who may have found this book when hunting/training is what would be the best mobs for me to try. I have read that Skeles and Coolie zombies both drop the book and would like to know from people's personal exp what seemed to work best for them. Thank for any tips/suggestions.