Likely to be a bug and fringe case of the combat system. In my case, the battle ended within less than a minute with myself and Diabolus killing him. The Stranger will heal only once, and most human enemies will use potion and heal about two or three times, after that they will break combat and run away and be killed. It does happen occasionally that you will encounter storyline plot enemies who have endless full heal because they are meant to surrender, say some last words before they are killed by you or require you to do something after you find out that they are invincible. If you cannot seem to kill an enemy because they seem to heal more than 3 times or so, try loading game and stick to melee with your companions. (There are exceptions on certain boss enemies where they are meant to be invincible until you do something) Some spells that do gradual damage such as locust swarm tend to cause this bug to happen. Try to recruit Diabolus as your first companion to help you with the fights as it is relatively easier to manage combat with companions fighting with you rather than going solo. (Search the town dungeons below the Sheriff's office)