This can easily be obtained if you go for the following quest trophy:
PK happens out in the field, sealed within a Battle Arena (blue arena that flashes with activity). You might fight regular enemies inside, or sometimes you will find that a player is harassing another. For the quest trophy mentioned above, you have to at least kill 50, so it isn't a far enough reach that five of them would brush against your blades.
If you're wanting an area with an immediate battle arena, go to (Delta) Starting Your Berserker. It is Level 1 and the arena is just south of the load-in portal. In and out.