During a regular story playthrough, a good spot to farm these kills is in Chapter 9.
After the initial Jet Ski run, Elena will drop you off saying that "you gotta get to that tower". Immediately fight off the three pirates. Now, just swim the flooded alley running into the end. By not getting out of the water, you will simply 'hang' there. Equip a gun, aim and shoot. Allow yourself to be killed and you will spawn at the point where Elena dropped you off but without the initial three pirates. Repeat until Trophy pops.
This is a good time to also work on the Grenade Hangman Trophy - kill 10 with grenades while hanging from a ledge.
As all the other guides suggest, manually save your game to preserve your medal points.