I didn't play assassin nor ranger for enough time to know what is the best stigmas and skills rotation for them , however there are some tactics which are basic in all classes like (silence cover)
I didn't play gunner before honestly i dont even play much with any alts , i'm the one class guy xD I found this video , it might be helpful for you: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-BwDlJ7bYE5E.html