Much like this question's author, I'm interested in gaining good XP at a high level. But rather than doing challenges (which often get done in the course of normal playing, especially the 'kill X of Y' type) I'd prefer to do it specifically by fighting enemies.
George Stocker mentioned in his answer that Motorhead respawns every 15 minutes, but surely he can't be the only unique enemy/boss who respawns. Skyscraper giving 23k XP would be great but as far as I know he's a one-off. It seems that an exhaustive list of respawning enemies would be quite useful.
Is there a list of farmable enemies, and where to find them?