Does the man have time to do anything else but write hundreds of amazingly specific, thoroughly researched and totally helpful manuals? I bet he has a book entitled 'So You're a Reincarnated Demon Lord...' that helps you deal with things like enemies from your past life out to kill you and the traumatic flashbacks to your last death. It probably doesn't sell very well outside Muspelheim and Hel, but them's the breaks in the publishing biz when you specialize.