After killing all the enemies in an area, it gets kinda lonely out there. You can run for miles without a single skull to cleave, because they simply won't respawn on their own.
I've found a solution to that problem.
Get your hands on a resurrection spell. Go kill some MOBs somewhere. Resurrect the buggers. As long as you are near, the MOBs remain allied to you, but... if you walk away and leave the area, the MOBs seem to remember that you are actually their enemy. Go back and slay them again to receive the exact same amount of XP you got the first time
I have also found that the special insane skeletons in the Dark Forest make some pretty nice drops upon dying. You've probably guessed it... Along with the XP, you get more nice goodies from them, over and over again.
Resurrect your poor victims, folks. They really deserve a second chance. To die and feed your everlasting hunger for XP, goodies and skills WOOHAHAHAHAHAAAA:mischief: