NINJA GAIDEN Σ

This tracks kills across all Story Mode playthroughs and Mission Mode. If you are only playing the story, this will likely unlock near the end of your Hard playthrough, assuming that you fight all of the Ambushes as Rachel both times. These alone on a Normal and Hard playthrough account for 420 of the 500 kills required, with there being enough smaller fights to make up the difference.

If you are playing Mission Mode as well, this will come even sooner, as each mission must be cleared 4 times to get the platinum coin, and will often contain upwards of 30 enemies per playthrough, mission depending. As such, there is no need to grind out kills in Mission Mode,as there are 500 here alone across all of the Rachel misssions.

As for killing enemies as Rachel, her best combo is X, X, X, X, which will kill man sized targets more often than not. Once there is loose essence from this, or uses of Sorcery, try and keep a non stop chain of ETs and UTs going, unless your opponent is an Ogre or Berserker. They should be dealt with by making distance and charging a UT, releasing when the Ogre gets close, and by counter attacking only for a Berserker.