Use the same technique as i have explained for for farming a level 10 NPC ( see http://evonytricks.blogspot.com/2010/07/farming-npc-lvl10.html ), but instead of waiting 8 minutes between attacks just keep sending troops as quickly as you can.
To be safe, send 3-4 times as many casualties as you are getting in subsequent attacks.
NOTE: You must have an empty city slot to conquer this NPC.
