This achievement is obtained by killing 50 different players in PvP worlds, and counts as 20% of the Master Killer achievement.
Master Killer Achievement
Kill 250 different players in PvP worlds.
To find PvP worlds, use the world teleport panel. Tap or click "Experiences," then "PvP." Very often, using the "Popular" tab will also display a PvP world or two.
You can use bombs to kill yourself in a PvP world and it will count 2% towards the initial achievement.
Killing people in non-PvP worlds (for example, with bombs) does NOT count and in fact will cause you to get karma. In addition, killing people in non-PvP worlds is rude, and if you do it too much, it can get you banned. Don't kill people in non-PvP worlds.
Killing the same person over and over will not count towards this achievement, and is also super rude.