When you hover over the npc's in the list, pick on those that have least resistance to your weapon (toughness for melee and reflex for range) and least aim / dodge. Those are the most likely candidates to KO.
This is a great tip. So are some of the others out there. To summarize:
1. get your duel difficulty down as low as you can stand. I went all the way to 0 by being KO'd. I took off all my gear to guarantee success. Once my energy was back to 5 I did it again. Over and over until you get duel difficulty to drop to 0 or 1. Sleep it off and build your HP and energy back.
2. Set your aim for all head shots and your stance to not dodge.
3. choose your NPCs carefully and patiently. Low reflex for range and low toughness for melee. And most importantly (depending upon the damage your weapon can bring) low HP.
4. Don't try to get to 10 in one day. Everytime you win your duel difficulty will go up and the NPCs will get higher HP, reflex and toughness. Probably 1-3 KOs a day is a good target.
I have completed this part on one world and I have 7/10 KOs and 4/10 KOs on another. (I figured out I should be using a melee weapon on the 4/10 world or I would have been higher.)