Over the course of The Screwtape Letters , the state of patient’s soul fluctuates

as he experiences a conversion, doubt, dangerous friendships, war, love, and
finally, in death, oneness with God. What major strategies does Screwtape use
to tempt the patient into the Devil’s camp? Why do these temptations fail?

Have you read the book? I highly recommend it. This does not "give" you the answer to this question (you have to read the book), but may help you understand it:

http://www.bookrags.com/studyguide-the-screwtape-letters/#gsc.tab=0