I keep trying to beat this fight at the end of The Championship Fight quest and I can only get him down to about half health before he quickly runs back to where he is at the beginning of the fight and his health recharges very rapidly until the bar is full. He does this repeatedly while I am fighting him. Am I doing something wrong or is my game broken?
Mobs can only go so far from where you initially engage them otherwise they will hit their 'leash' and reset. Try kiting mobs in circles rather than just dodging backwards all the time.
The reasoning behind this behaviour is to stop what happened in certain early MMORPG's where people would end up with 'train's of creatures trailing behind them until eventually they died, usually taking several other people along with them.