In Vizima near the sewer entrance, you meet the "con artist" who gives you a key to a crypt in the sewers. In that crypt are at least 6 graviers plus other, lesser undead. The graviers are worth several hubdred xp per pop (depending on your level). When you're done, exit the crypt, then immediately renter. They're all back. It's a pretty big crypt, and the undead are in different positions each time you go in, so it's a fairly interesting "grind" area. First time through got me about 6,000 Xp. By level 28 it was down to about 2,500, but that's still pretty good. At one point I had over 100 gravier heads in my pack.