This is by far the easiest method.
1: Do the ESCAPE ADVANCED MISSION.
2: Progress through the mission until you get to tumbleweed.
3:you will notice on top of the hill over-looking tumbleweed there are some conviently placed cannons there. take out the people at the hill with your buddies and then use one of the cannons.
4: Now just kill the enimies below. Aim at the town area first theres about 8 or 9 people there, then you will see about 5 or 6 people around the church, shoot them.
AND OOH WHATS THAT? ... ACHIEVEMENT UNLOCKED!!!
(REMEMBER THESE KILLS HAVE TO BE IN A CHAIN)
this can also help you towards the mowing them down achievement getting 500 kills with a mounted weapon.