Well at least for me.. I just can't seem to get them.  Every time I try to shoot dudes it's neither too much or not enough. How can you determine when they are within "one punch kill" range?
 
Anybody got some tips they can throw my way? 

1. Run towards pirate
2. Shoot said pirate
3. Punch

Best thing to do, as the stats are all cumulative, is repeatedly start the game again one easy and do it to the pirates that board your boat in  the beginning scene

put it on easy, then it just takes a couple of pistol shots when you're running towards them then punch-a-roo.