In Red Dead Redemption, can I kill people near the end when coming out of the barn

That does seem to be the fastest way. You could also try riding as a passenger in one of the stagecoaches set up on the edge of one of the towns.

Alternately, in free roam you can go to one of the wagon wheel markers in a settlement, but that works pretty much the same way.

I had trouble with dueling too. Perhaps try shooting where your crosshair lights up red. Also wait until dead-eye automatically engages and it tells you to draw. (?)

You can also try disarming the guy by shooting him in the hand. Some people, usually story related, cannot be disarmed, as such aiming for the hand will only get you killed instead, which can be frustrating.

Rockstar discussed it a little here, hopefully this is helpful: http://www.rockstargames.com/newswire/article/5991/rockstar_game_tips_dueling_like_a_pro_in_red_dead_redemption.article