(1) There are several extremely valued beings in the game. These beings are well known so that I don't think I need to list them
Well, it's unfair sometimes but XPs are XPs ...
(2) Make sure that you are Paladin when you reach guild level 5 in the Temple and get the Crusade trait. All undeads will give you twice as much XPs than normally. Shurugeon Castle or Skull Castle become bonanzas of XPs for you.