Call of Juarez

Although there is another guide here it doesn't give any tips. Make sure you select Gunslinger. If the other player goes Gunslinger and you don't and they drop dynamite for you, no matter how much you try this won't unlock (trust me :-))

Make sure you are both Gunslingers so you can refill on their dynamite (the other player should equip the dynamite so that they drop it). Find a wall and get your opponent to stand against it. Aim for their crotch and throw it. The dynamite should hit them and take off a little damage. The explosion should then kill them. Repeat this 3 or 4 more times to unlock the achievement (Took me 5 times, but others report 4 times so I assume I did something wrong first time)

We found if you didn't hit them, then it wasn't a guaranteed kill. Also the reason for the wall was because if we just did it in the open the dynamite bounced everywhere messing up our kills.

This can be done with a friend. Choose both the gunslinger because then you have dynamite. Kill your opponent five times in a row with the dynamite without missing. Pick up the dynamite that the opponent drops when he's killed.